Add rc-testing workflow, update actions versions.

[mkelley]

Add a workflow file for manual testing of dependency release candidates.

[github-actions]

Thank you for your contribution to sbpy, an Astropy affiliated package! 🌌 This checklist is meant to remind the package maintainers who will review this pull request of some common things to look for.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the sbpy coding guidelines?
• Are tests added/updated as required? If so, do they follow the Astropy testing guidelines?
• Are docs added/updated as required? If so, do they follow the Astropy documentation guidelines?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions. Also see instructions for rebase and squash.
• Did the CI pass? If no, are the failures related?
• Is a change log needed? If yes, did the change log check pass? If no, add the "no-changelog-entry-needed" label.
• Is a milestone added?

[codecov]

Codecov Report

❌ Patch coverage is 25.80645% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.33%. Comparing base (e720e46) to head (3fd3e42).

Files with missing lines	Patch %	Lines
sbpy/data/obs.py	14.81%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
- Coverage   82.11%   81.33%   -0.78%     
==========================================
  Files          53       53              
  Lines        4339     4340       +1     
==========================================
- Hits         3563     3530      -33     
- Misses        776      810      +34     

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