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.
When there is no externally allocated baseline, we should
do the same thing that GtkBox does, and determine one from
the children that want baseline alignment.
This commit adds a GtkCenterBox::baseline-position property
with setters and getters.
