v7.1: remove LibGit2Sharp and native binaries

[arturcic]

Summary

Remove LibGit2Sharp and all shipped native Git binaries in GitVersion v7.1, after the managed backend ships as the v7.0 default under #5135.

Part of #5031. The release and selector lifecycle is governed by #5135, with final environment-variable cleanup in v8 tracked by #5136.

Remove the legacy backend

• Delete src/GitVersion.LibGit2Sharp and new-cli/GitVersion.Core.Libgit2Sharp.
• Re-point any remaining source links or consumers to the managed backend.
• Drop LibGit2Sharp and native-binary package references.
• Add packaging assertions proving shipped packages contain no runtimes/**/native/* entries.
• Register the managed backend directly at production composition roots.

Remove dual-backend validation scaffolding

• Collapse the unit-test git_backend matrix to managed-only execution.
• Collapse Docker and artifact tests to one managed-backend path.
• Remove backend inputs and environment forwarding from reusable workflows/actions and build utilities.
• Delete dual-backend parity tests and libgit2-only test visibility grants.
• Remove corpus tooling whose purpose is comparing managed and libgit2 results.
• Keep managed-only tests that originated from parity work.

Retain the v7 selector compatibility shell

Do not remove recognition of GITVERSION_GIT_BACKEND in this issue:

• GITVERSION_GIT_BACKEND=libgit2 fails with an actionable message that the backend was removed in v7.1.
• GITVERSION_GIT_BACKEND=managed remains accepted as a no-op throughout v7.x so explicit v7.0 opt-in automation keeps working.
• The selector and its compatibility diagnostics are removed in v8 by Remove feature-selector environment variables #5136.

The implementation no longer performs backend selection after this issue; only the v7 compatibility validation remains.

Documentation

• Update the managed-Git design/status and v6-to-v7 migration documentation to show managed as the v7.0 default and LibGit2Sharp removal in v7.1.
• Document the v7.1 error for GITVERSION_GIT_BACKEND=libgit2 and the v8 removal tracked by Remove feature-selector environment variables #5136.

Acceptance criteria

• Shipped packages contain no LibGit2Sharp assemblies or native Git binaries.
• Production and CI use only the managed backend.
• No dual-backend selection or comparison infrastructure remains, apart from the v7 environment-variable compatibility validator.
• Legacy selection fails clearly and explicit managed remains harmless in v7.x.

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.

Branch additions relevant to backend removal

• src/GitVersion.Core/Git/GitBackend.cs now defaults to managed Git and shares trimmed/case-insensitive/blank-default validation through src/GitVersion.Core/FeatureSelector.cs, using WarningException. In v7.1, replace the libgit2 selection path with actionable removal guidance; keep explicit managed accepted until v8.
• src/GitVersion.App/FeatureSelections.cs captures the backend before CliHost.cs registers it and GitVersionApp.cs logs the captured value. Remove the libgit2 registration/logging branch while retaining truthful managed diagnostics and stderr isolation.
• Review new-cli/GitVersion.Common/GitVersion.Common.csproj: the new shared helper is explicitly source-linked because the Git backend selector is source-linked. Keep that dependency buildable until Remove feature-selector environment variables #5136 removes the selector/helper.
• Update src/GitVersion.Core.Tests/Core/GitBackendSelectorTests.cs: replace libgit2 success cases with removal-error cases, retaining managed/default/empty/padded/case-insensitive and unknown-value validation.
• In src/GitVersion.App.Tests/FeatureSelectorTests.cs, reduce CompositionRegistersSelectedImplementations to managed-only registration after this cleanup. Coordinate v6 parser/configuration cases with v7.1: remove legacy argument parser and v6 configuration runtime #5188. Adapt SelectionLogUsesCapturedValues without losing the captured-state assertion.
• In src/GitVersion.App.Tests/ExecFeatureSelectorTests.cs, retire libgit2 runtime/migration variants in SelectorCombinationsCalculateAndLogWithoutPollutingOutput and MigrationRetainsYamlStdoutWithEitherConfigurationAndBackend; add an actionable libgit2-removal stderr case to InvalidSelectorsFailBeforeHelpWithActionableStderr. Preserve machine-readable output and default-selection tests.
• Remove backend matrices from workflows/artifact tests only with the implementation cleanup. build/common/Utilities/DockerContextExtensions.cs now correctly documents the absent-selector default as managed.

The v7.0 branch has tests for every parser/configuration/backend combination and concrete registered implementation. Remove the obsolete alternatives rather than deleting the whole selector/output-isolation suite. Selector variables and common validation are removed only in #5136 (v8).

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.