[Deprecation] `Facter::Util::Resolution` Class-Level `exec`/`which` Exposure

[silug]

Summary

Facter::Util::Resolution publicly exposes compatibility command-execution methods and marks that compatibility block as deprecated.

Evidence

• lib/facter/custom_facts/util/resolution.rb:30 marks compatibility exposure as deprecated.
• lib/facter/custom_facts/util/resolution.rb:33 exposes public :which, :exec from Facter::Core::Execution.
• Inline comment states this was a compatibility bridge after extraction to Facter::Core::Execution.

Proposed Change

• Remove deprecated class-level exposure of which and exec on Facter::Util::Resolution.
• Require callers to use Facter::Core::Execution directly.

Compatibility / Risk

• Medium risk for older custom facts invoking Facter::Util::Resolution.exec/which.
• exec removal aligns with separate deprecation issue for Facter::Core::Execution.exec ([Deprecation] Facter::Core::Execution.exec #88).

Implementation Notes

• Audit internal and fixture call sites for Facter::Util::Resolution.exec/which.
• Keep with_env exposure only if still required and intentionally supported.
• Coordinate rollout with Execution.exec removal to avoid partial breakage.

Acceptance Criteria

• Facter::Util::Resolution no longer exposes deprecated exec/which compatibility methods.
• No internal code path depends on these compatibility methods.
• Documentation directs consumers to supported execution APIs.

Suggested Tests

• Custom fact execution tests using supported API paths.
• Negative tests for removed compatibility methods if explicit failure behavior is desired.

[silug]

Marked deprecated in documentation in 757b458.

This also likely needs an explicit deprecation warning like #88.