"What's New" for #3762 (unpinning Cartopy, Matplotlib, Proj)

[trexfeathers]

#3762 was created before the ongoing revamp of the documentation, so does not include the architecture for the current "What's New" process. There has been much discussion of the future complication that can be caused by rebasing/merging from master onto a PR, so I suggest that we instead merge #3762 then write the relevant PR after merging (when our changes will be present alongside the latest "What's New" architecture).

Note from the Contribution categories that the "What's New" should also include a section for internal changes such as the testing changes we are making have made.

[trexfeathers]

ping @stephenworsley

[trexfeathers]

It's important that this work also addresses #3759 (comment)

[trexfeathers]

Closed by #3811