Convert hooks to OOP hooks with attributes

Should probably kill _group_relation_type_manager() now, but that requires a CR.

Should go over each hook manually to see if it still works.

Follow-ups:

• Deprecate _group_relation_type_manager() (v2/3) and remove in v4.
• Add aliases to our services so they can be autowired (v2/3/4)

Pushed a commit that uses hook ordering and the new theme initial preprocess callbacks. The entire module file is gone now and I've bumped minimum required version to 11.2. That last bit may make tests fail as we don't have an official 11.2 release yet.

Tests are failing on me forgetting to convert tokens.inc and views.inc, also need to tackle submodules still.

All green for next minor, so will commit after one final round of reviewing the code and threads.

kristiaanvandeneynde → credited berdir → .

kristiaanvandeneynde → credited nicxvan → .

Going to commit this in its current shape. Can still make changes before release if the meta in core evolves regarding best practices.

Automatically closed - issue fixed for 2 weeks with no activity.