AXes subclasses should override clear instead of cla

For clarity, axes.Axes.clear is now preferred over Axes.cla. However, for backwards compatibility, the latter will remain as an alias for the former.

For additional compatibility with third-party libraries, Matplotlib will continue to call the cla method of any Axes subclasses if they define it. In the future, this will no longer occur, and Matplotlib will only call the clear method in Axes subclasses.

It is recommended to define only the clear method when on Matplotlib 3.6, and only cla for older versions.