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.