Use #[IgnoreDeprecations] instead of #[Group('legacy')] - again

MR is ready for a review.

Trivial MR.
Issue summary and title match the diff.
Pipeline is green, other than the PHP 8.5 warnings.
Tagging to be smashed.

Nothing else to do or improve that I see, unless we want a follow-up to add something to automatically flag further regressions to using the legacy group?

#6: 📌 Deprecate expectDeprecation(), use PHPUnit's expectUserDeprecationMessage() instead Postponed will cover that in the sense that after it, #[Group('legacy')] will just be a group like any other and any deprecation thrown during the test will make the test fail. So I wouldn't do anything additional right now; let's play whackamole when they pop up.