Remove LibGit2Sharp and native binaries (Phase E)

[arturcic]

After the managed backend has been default for several releases without regressions: delete src/GitVersion.LibGit2Sharp and new-cli/GitVersion.Core.Libgit2Sharp, re-point new-cli source-linking at the managed reader/adapter, drop the package reference, add a packaging assertion test that shipped nupkgs contain zero runtimes/**/native/* entries, update docs.

Also remove the dual-backend validation scaffolding

With libgit2 gone there is a single backend, so the machinery that exists only to run and compare both implementations should be removed too:

• Backend selection — the GITVERSION_GIT_BACKEND environment variable, GitBackendSelector/GitBackend (in GitVersion.Core), and the selection branches at the composition roots (CliHost, GitVersionCoreTestModule) collapse to registering the managed module directly.
• Dual-backend CI matrix — drop the git_backend: [libgit2, managed] dimension and the GITVERSION_GIT_BACKEND env in .github/workflows/_unit_tests.yml (and revert the matrix.git_backend == 'libgit2'-gated coverage/summary conditions).
• Parity tests — delete DualBackendParityTests and GitBackendSelectorTests (and the libgit2 InternalsVisibleTo grants that exist only so those tests can open the libgit2 backend directly).
• Parity corpus tooling — remove the corpus diff scripts (build/parity-corpus.ps1, tests/scripts/backend-parity-corpus.sh) that cross-check the two backends on real-world repositories.
• Docs/migration notes — drop the GITVERSION_GIT_BACKEND entries from the v6→v7 migration guide and env-var tables.

Keep the managed-only tests that happen to have grown out of parity work (e.g. GitVersion.Git.Managed.Tests) — only the comparison/selection scaffolding goes.

Part of #5031 — see docs/design/managed-git-migration.md for the full design.