Update book and templates with feature freeze

[blyxyas]

This PR announces the feature freeze period talked about in #14364

1. Add the page to the book
2. Modify the in-Github templates.
3. The third commit (to be squashed into the first) rolls the date mentioned 6 weeks (so, starting on May 9th and ending on August 20th). This gives us a comfortable buffer to make choices for this period.

We have a pending discussion on the #14364. So having some more time to make choices is very nice. I'm also preparing a Github action for detecting new lints and posting a comment about the feature freeze (being worked on in another branch)

Something I'd like comment on is the date formatting. I'm not sure if "May 9th to the first of August" is the correct way of writing this information in a book 😅.

changelog: Announce the feature freeze from May 9th to the first of August

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[blyxyas]

I've managed to create a script for a GitHub actions that detects lint additions and adds the PR to a milestone. But that will get a specific PR when the time comes (See a demo here)

I've also created a to-be-used FEATURE FREEZED milestone. This is ready to be reviewed.

[samueltardieu]

Something I'd like comment on is the date formatting. I'm not sure if "May 9th to the first of August" is the correct way of writing this information in a book

Maybe this is the time where a review from copilot could be useful.