replace BACKEND_SPECS with matplotlib.backends.backend_registry #5138

While updating the BACKEND_SPECS in #5135 , I noticed that matplotlib 3.9 introduced a matplotlib.backends.backend_registry that could be used to replace BACKEND_SPECS entirely and offload the backend details to matplotlib.

e.g., could add the following to get the FigureCanvas and FigureManager classes in _get_canvas_classes

if matplotlib.__version_info__ >= (3, 9):
    from matplotlib.backends import backend_registry

    mod = backend_registry.load_backend_module(key)
    FigureCanvas = getattr(mod, 'FigureCanvas')
    FigureManager = getattr(mod, 'FigureManager')

Only thing I'm a little uncertain of is the FigureManager -- from what I understand, the above code pulls out FigureCanvas and FigureManager classes that are aliased to the specific implementation for each backend and there should always be a FigureManager specified (which seems to have been the case for quite a while now... since matplotlib/matplotlib#8773). Any reason to preserve yt's behavior for the backends where None is specified for a FigureManager in BACKEND_SPECS? I think no, but wanted to get a second opinion.

here's the relevant bits of code i'm referring to:

and

Also, I don't see a huge benefit to putting this change in now vs waiting til yt's min matplotlib version is 3.9, but wanted to get these thoughts down while they're fresh.