The idea is that GTK+ 4 will be an epoch, API-wise.
Everything that was around for 4.0 has been there
since the beginning of the epoch and doesn't need
markers.
Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.