[v4] PHP 8.4 support

[distantnative]

• Stops using PHP 8.1 for CI backend tests

Changelog

Enhancements

• Support for PHP 8.4

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion

[lukasbestle]

I think most of the CS fixes will create merge conflicts with v5. E.g. in v5 we have replaced the something|null $param = null params with something|null $param and made some fixes to docblocks where CS fixer now again wreaks havoc.

If we merge this as is to ensure PHP 8.4 compatibility in v4, we should probably not continue to merge back v4 changes into the v5 branch, otherwise it will be a merge conflict hell. Alternative could be to backport the CS changes manually from v5.

[distantnative]

@lukasbestle We made nullable parameters non-optional?

This got created as Basti mentioned the user demand for 8.4 support in v4 as well. I think we'll need to tackle the merge conflict once. I think it'll be not too bad (as we've merged regularly, merging into v5/develop after merging this PR we can go with the v5/develop version in pretty much all conflicts). But while we probably won't have a lot more to merge from v4 to v5 I wouldn't want to rule it out in general.

[lukasbestle]

Not in general, but in some Str methods (breaking change) and also in those cases where the parameter is not in the last position, which are those lines changed by this PR. Those parameters are never optional, which is the reason why PHP 8.4 deprecates the type $param = null syntax in favor or type|null. We had discussed that in #6398 and implemented it with #6401.

Looking back, it would have been better to make those changes in v4 and not v5. But now it's like it is.

[distantnative]

If we merge develop-minor into v5/develop right before and right after merging this PR and then go with the v5/develop version for any merge conflict, we should be fine, in terms of merge conflicts.

In terms of making those changes for v4 as well of course not. But maybe that's ok that v4 doesn't get those enhancements but just most basic 8.4 support.