refactor(manage-merge-queue, check-merge-safety): point users to native GitHub merge queue

[danadajian]

Summary

• Deprecates the manage-merge-queue and check-merge-safety helpers in favour of GitHub's native merge queue, which now provides the same capabilities as both helpers natively.
• Both helpers continue to work — the deprecation is non-breaking — but callers will now see a warning in their workflow logs directing them to migrate.
• The README sections for both helpers are marked with ~~strikethrough~~ and a (DEPRECATED) label, following the existing convention used in this repo.

Changes

• manage-merge-queue and check-merge-safety emit a core.warning at the start of each run with a link to the GitHub merge queue docs.
• README headings for both helpers are struck through and annotated (DEPRECATED), with a blockquote linking to the migration docs.

Test plan

• Trigger a workflow that uses manage-merge-queue — confirm a deprecation warning appears in the job log.
• Trigger a workflow that uses check-merge-safety — confirm a deprecation warning appears in the job log.
• Review the README and confirm both sections show the deprecation notice with the correct link.

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🎉 This PR is included in version 1.87.1 🎉

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