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.
GdkContentProvider is the object that represents local data in the
clipboard.
This patch only introduces the object and adds the clipboard properties,
it does not yet provide a way for the actual implementations to access
it.
The only access that is implemented is the local shortcut GValue access.
