Update readthedocs ubuntu version

[jmafoster1]

ReadTheDocs announced that the deprecation of the Ubuntu 20.04 LTS build image. This PR updates the workflow to use 24.04.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	32		0	1.06s
✅ PYTHON	pylint	32		0	5.93s

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.90%. Comparing base (d4765fa) to head (f4361b5).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #380   +/-   ##
=======================================
  Coverage   97.90%   97.90%           
=======================================
  Files          27       27           
  Lines        1572     1572           
=======================================
  Hits         1539     1539           
  Misses         33       33           

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[f-allian]

Keeping note of this - we might have to return to this PR in the future as I remember Python 3.11 being problematic for RTD but we won't know this until it's been deployed.