Implement MSC4210: Remove legacy mentions

[tulir]

Implements https://github.com/matrix-org/matrix-spec-proposals/pull/4210

Pull Request Checklist

• [x] Pull request is based on the develop branch
• [x] Pull request includes a changelog file.
• [x] Code style is correct (run the linters)

[tulir]

Just realized I forgot to modify the list of push rules sent to clients

[tulir]

Now the implementation should actually be complete

[tulir]

Complement seems to have some flaky delayed events tests 🤔

[CLAassistant]

All committers have signed the CLA.