DOC: don't use deprecated collections attribute

[rcomer]

Rationale

I just spotted this while grepping for something else. The ContourSet.collections attribute was deprecated at mpl 3.8. We could now just call set_visible on the ContourSet itself, but that will break with older Matplotlib versions. I have checked that the current suggestion works with Cartopy main and mpl 3.8.2, and also with Cartopy 0.21.1 and mpl 3.6.3.

Implications

[rcomer]

I guess another option would be to just have two commented-out lines, one each for older and newer Matplotlib versions. Is it more efficient at draw time to have the visible flag set to False than the color set to 'none'?

[greglucas]

I think there are some shortcuts if artist.get_visible() == False, so it likely is a bit faster. This is only an example though, people can expand from here themselves.