Address CI failures from upstream Matplotlib changes

[kafitzgerald]

Addresses several of the failures we're seeing while testing with a dev version of Matplotlib (3.10.0.dev).

These stem from deprecations following earlier Matplotlib changes with ContourSet and legend.

Relates to #620

There will be a lingering problem related to hatch color that this PR does not address so I'm leaving the associated issue open for now. There's also an open issue (confirmed bug) on Matplotlib.

[kafitzgerald]

This is proving trickier than I thought. In particular, the changes for NCL_coneff_11.py which attempt to adapt the code to update the hatch colors and linewidths after the Matplotlib changes aren't working

I tried a few other things and dug into the docs a bit, but am not having much luck. It seems like the hatching related code isn't the most straightforward or robust and we might be hitting on some related challenges with this.

I'll come back to this later (and likely log an issue over on Matplotlib), but if anyone else has ideas I'm open to suggestions here.

Some additional details...

The deprecation of ContourSet.collections and then removal in 3.10 is what's causing upstream CI (Matplotlib 3.10.dev) to fail. Relevant release notes:
https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.8.0.html#contourset-collections

We iterated over collections to help set colors for the hatching in NCL_coneff_11.py and customize several other examples. I've been able to successfully make changes to the other examples, but for whatever reason the hatch properties seem to be problematic.

[jukent]

It seems like we were working around things in matplotlib that were changed upstream because these changes stop the failure and are more concise.