Create a 2.0.0 release

As mentioned in #3023170-57: Test for compatibility with Drupal 9 → , sooner or later we should probably create a new major release, removing all the currently deprecated code in this module. (Or maybe just the not-too-recently deprecated code?)
And as semver is now finally supported for contrib → , this major release should probably be 2.0.0.

Also, we might want a 2.x branch at some earlier point to move issues to it – but I guess I can just add that when the need arises. (As we normally try to implement new features in a backwards-compatible way, I currently can’t think of a reason.)

Anyways, when should we create this release? Opinions very much appreciated!

Ideas

• Require PHP >= 7.3
• PSR-18 event dispatching
• Add type hints (see also #[\ReturnTypeWillChange] attributes)