Remove matplotlib version pin

[kafitzgerald]

PR Summary

Remove matplotlib version pin now that most contour labeling issues have been addressed upstream (e.g. matplotlib/matplotlib#27045)

Also thins the number of contour labels added to the plot_contour_labels usage example to make it look a bit cleaner and remove problematic locations that hit upon the remaining bug in matplotlib contour labeling.

We could opt to leave some of these problematic locations in and wait until there's a fix in matplotlib to remove the version pin, but geocat-examples will still catch this particular issue. Given that this is a bit of a an edge case, I think the benefit of removing the version pin from geocat-viz at this point outweighs the risk to users.

Closes #160

PR Checklist

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[kafitzgerald]

Apparently the link for mamba installation instructions changed and that's what the link checker caught.

I'll just update that in here as well.