[release-16.0] Automatically add milestone to new Pull Request (#12759)

[frouioui]

Description

This is a backport of #12759.

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• If this is a change that users need to know about, please apply the release notes (needs details) label so that merging is blocked unless the summary release notes document is included.
• If a test is added or modified, there should be a documentation on top of the test to explain what the expected behavior is what the test does.

If a new flag is being introduced:

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If a workflow is added or modified:

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Bug fixes

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Non-trivial changes

• There should be some code comments as to why things are implemented the way they are.

New/Existing features

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Backward compatibility

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