Fix working with Matplotlib 3.9

[dl1jbe]

Starting with matplotlib-3.9 voacapgui fails to draw diagrams for P2P and Area plots with following problems:

• register_cmap() do no longer exist
• AxesGrid() does no longer allow an empty label_mode parameter

After fixing it shows that diagrams in Area plot have grid label on all four sides.

The provided PR fixes these problems and take care of some deprecation warning for GeoAxes projection.