Update repo RM role policy regarding EOL branches

[hugovk]

As detailed in PEP 101:

• Freeze the state of the release branch by creating a tag of its current HEAD and then deleting the branch from the CPython repo. The current HEAD should be at or beyond the final security release for the branch: [...]

• If all looks good, delete the branch. [...]

Let's update the policy to match.

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📚 Documentation preview 📚: https://cpython-devguide--1483.org.readthedocs.build/