Remove Google Group link

[rcomer]

Rationale

The Google Group is not much used any more, and over in Iris we have been discussing retiring it (SciTools/iris#3860). Do you have any objections to that? Iris have decided to use GitHub Discussions instead. Possibly Cartopy doesn't need anything to replace it as there seems to be quite a lot of Cartopy activity on StackOverflow.

Implications

One less place for users to ask questions, but also one less place that support people need to keep on top of.

[greglucas]

If Iris is moving away from it, seems like a good idea to remove it from Cartopy's docs.

[lbdreyer]

It looks like google groups are also mentioned: https://github.com/SciTools/cartopy/blob/main/.github/SUPPORT.md so that may be worth updating at the same time?

[rcomer]

Nice spot @lbdreyer! I have removed that link and also copied the gitter link from the README, as I assume they should be consistent.

[dopplershift]

That's fine with me.

[QuLogic]

We should maybe point to the Matplotlib Discourse which has a Cartopy category: https://discourse.matplotlib.org/c/3rdparty/cartopy/19

[rcomer]

Since the scope of the 3rd Party section on Matplotlib's Discourse seems pretty similar to what was previously written for the Google Group, I just reused that text.

[rcomer]

Hi @SciTools/cartopy-devs, is there any more I should do for this one? The Google Group has now been retired.