Add a policy on supported test dependencies

[dstansby]

It's probably useful to have this whilst fighting the 🗑️ 🔥 that is our CI at the moment. I think 12 months strikes a good balance between us not having to support too many versions and others (especially packagers) having to have the latest version available.

[ConorMacBride]

Merge or discuss more later?

[sunpy-backport]

The backport to 3.0 failed:

Commits ["5b49179b24aa578bbee9ad9c16219ce03de02e73"] could not be cherry-picked on top of 3.0

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport 3.0
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 5b49179b24aa578bbee9ad9c16219ce03de02e73
# Create a new branch with these backported commits.
git checkout -b backport-5926-to-3.0
# Push it to GitHub.
git push --set-upstream origin backport-5926-to-3.0
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is 3.0 and the compare/head branch is backport-5926-to-3.0.