Remove feature-selector environment variables

[arturcic]

Summary

Remove the v7 compatibility selector environment variables in GitVersion v8 after their legacy implementations have been removed in v7.1.

This is the final cleanup following the v7 feature-selector alignment issue.

Progress

• Complete prerequisite v7.0: default to v7 parser, nested configuration and managed Git #5135 — v7.0 selector alignment.
• Complete prerequisite v7.1: remove legacy argument parser and v6 configuration runtime #5188 — v7.1 legacy parser/configuration runtime removal.
• Complete prerequisite v7.1: remove LibGit2Sharp and native binaries #5040 — remove LibGit2Sharp and native binaries.
• Remove all three compatibility selectors and their selection abstractions in v8.
• Register the fixed implementations directly and retain gitversion config migrate.
• Add actionable diagnostics and tests for retired environment variables.
• Update v8 breaking-change and migration documentation.

Remove

• GITVERSION_ARGUMENT_PARSER_VERSION
• GITVERSION_CONFIGURATION_VERSION
• GITVERSION_GIT_BACKEND
• Selector types, parsing, validation, logging branches, compatibility diagnostics, and tests that exist only for these variables.

GitVersion v8 has one implementation for each concern:

• POSIX argument parser
• Nested calculation/output configuration
• Managed Git backend

Register and use those implementations directly rather than passing through feature-selector abstractions.

Behavior

• The three retired variables no longer influence execution.
• Prefer failing fast with a clear removal message when a retired variable is set, rather than silently accepting stale deployment configuration.
• Remove the per-selector informational log lines where they no longer communicate a choice. Retain ordinary diagnostic information where useful, such as identifying the managed Git implementation.
• Keep gitversion config migrate available for converting old configuration files; migration support does not imply runtime support for the v6 configuration format.

Acceptance criteria

• No production selection code reads the three variables.
• No compatibility selector abstractions or no-op selection branches remain.
• Packaging contains no LibGit2Sharp/native backend artifacts.
• Legacy argument parsing and legacy configuration loading remain unavailable.
• Tests verify actionable diagnostics when retired variables are supplied.
• v8 breaking-change and migration documentation lists the removed variables and fixed implementations.

Cleanup inventory from the v7.0 selector branch

The following inventory comes from arturcic:feature/v7-feature-selectors at 54adbb24e, the implementation of #5135. Implementation PR: #5189, opened from the fork after all four fork workflows passed on that commit. The inventory describes the PR branch; completion of #5135 remains a prerequisite until the PR is merged.

Production inventory for v8 selector removal

• Remove selection-only code in src/GitVersion.App/ArgumentParserVersion.cs, src/GitVersion.Core/Configuration/ConfigurationVersion.cs and src/GitVersion.Core/Git/GitBackend.cs after v7.1: remove legacy argument parser and v6 configuration runtime #5188/v7.1: remove LibGit2Sharp and native binaries #5040 have removed the legacy implementations.
• Remove src/GitVersion.Core/FeatureSelector.cs once it has no consumers, and remove its explicit source link from new-cli/GitVersion.Common/GitVersion.Common.csproj together with obsolete source-linked selectors. Build both solution trees to catch dangling links.
• Remove or simplify src/GitVersion.App/FeatureSelections.cs and its registration/logging through CliHost.cs and GitVersionApp.cs; register fixed implementations directly. Retain ordinary implementation diagnostics where useful.
• Update GitVersionCacheKeyFactory and any remaining configuration serializer/provider/override consumers of ConfigurationVersionSelector. Preserve configuration-version/cache compatibility where still necessary independently of environment-variable selection.
• Keep the minimal retired-variable detection needed for actionable errors, separately from runtime selection. Include the old GITVERSION_USE_V6_ARGUMENT_PARSER boolean in the retirement policy instead of leaving a dangling replacement message pointing at a selector that v8 has itself removed.
• Keep Program.cs clean stderr startup errors, migration's root-level --log-file support and ServiceCollectionExtensions machine-readable-output isolation. These behaviors must survive removal of selector-specific logging.

Tests to adapt rather than indiscriminately delete

• Retire success/normalization tests that exist only for the removed variables in GitBackendSelectorTests, ConfigurationVersionSelectorTests and FeatureSelectorTests. Replace their selector-routing assertions with fixed-implementation composition tests.
• Remove SelectionLogUsesCapturedValues when the selection snapshot is removed; retain any ordinary logging behavior under a suitable test.
• Adapt ExecFeatureSelectorTests: remove selector permutation setup from SelectorCombinationsCalculateAndLogWithoutPollutingOutput and MigrationRetainsYamlStdoutWithEitherConfigurationAndBackend, while retaining JSON, single-variable, show-config and flat-input migration output-isolation regressions.
• Rewrite InvalidSelectorsFailBeforeHelpWithActionableStderr for the v8 retired-variable policy. Cover all three retired variables, explicit formerly valid values, unknown values and empty values; diagnostics must name the retired variable and explain that only the v7 parser/nested configuration/managed Git remain, without offering v6/libgit2 or removed selectors as a workaround.
• Adapt DefaultsSelectV7AndManagedWithConsoleLogsOnStderr to assert fixed runtime behavior and any retained logging, without expecting informational choice messages that no longer represent a choice.
• Keep migration from flat v6 input supported and tested even though neither v6 runtime loading nor selector variables remain.

Sweep the branch's updated BREAKING_CHANGES.md, migration guide, CLI arguments, environment-variable reference and managed-Git design document. Describe retired variables as removals; remove temporary-fallback setup instructions from current v8 guidance while preserving historical v7 documentation where appropriate.

Output-isolation regression added during #5189 review

• Preserve the buildServer dimension in ExecFeatureSelectorTests.SelectorCombinationsCalculateAndLogWithoutPollutingOutput: JSON, single-variable and show-config stdout must stay free of diagnostics even when build-server output is also selected. This dimension is independent of the parser/configuration/backend selectors being retired.
• Retain consistent stderr routing for console logs in ServiceCollectionExtensions.ConfigureLogger. Build-server integration commands keep their existing output channel; diagnostic logs must not move back to stdout during cleanup.
• Keep CLI help (ArgumentParser and, until legacy removal, legacy_help.md) and the CLI/migration documentation consistent with stderr logging.