This PR collides with #41290 from @derrabus, I fear we might prefer closing it in favor of #41290, since return types must be added at the same time as arguments because of the PSR dependency...

We can finish this PR first and I'll happily rebase mine afterwards.

Can we? Because we support psr/cache v1, we cannot update argument types...

#41290 was about adding return types. For now, I'd skip the argument types coming from PSR Cache because that is a bit more complicated. For instance, the integration test suite we're using does not handle them well yet.

So, let's make this PR about adding parameter types to all methods except the psr/cache implementations and iterate further afterwards.