This affects methods like List.getFirst/getLast where it was decided
to keep them hidden but to make them visible on overrides like
ArrayList or LinkedList.
The goal is to prevent a breaking change since some implementations
like LinkedList had this method before JDK 21.
#KT-65438
^KT-58765
Now that JDK 21 was released as stable, we can remove separate test
configurations for testing the compiler with JDK 21 and reintegrate
the corresponding test data into standard test configurations.
Kirill Rakhman