[New Feature]: Warn before removing an entire package from repo

Description of the new feature/enhancement

Inspired by https://github.com/microsoft/winget-pkgs/issues/120261#issuecomment-1914362575

If a package only has a single version in the repo, and that is getting removed in a PR, the pipelines should warn the author / moderators in the PR. Most packages that have a single version in the repo are ones that use a "vanity" URL. Because of hash mismatches / 404 URLs, those tend to get auto-removed by wingetbot or other automations set up by community contributors.

This offers a bad UX to end-users that get confused as to why the package was entirely removed from the repo. In reality, the package may simply require an update to address the hash mismatch or URL issue. The validation pipelines should have ways to protect against this scenario. Retaining the package in the repository, albeit with an outdated hash / URL means that the contributors can report against the correct problem and request a package update.

Proposed technical implementation details (optional)

No response

Start by tracing the validation pipelines and PR checks that process manifest removals, then inspect how they determine whether a package still has versions in the repository. Define the warning behavior for removing the final version and verify it with a PR containing that case.