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.
A widget intended to offer contextual actions for a given view.
It allows packing children into the start or end as well as offering
a single centered child box.
https://bugzilla.gnome.org/show_bug.cgi?id=721665