[python/docs] Use ubuntu-24.04 (C++ 20) for readthedocs

[ryan-williams]

Issue and/or context:

• [c++] Use C++20 #3154 [sc-57301]
• RTD build failed after [c++/python/r] Migrate to C++20 #3331
• Test RTD build from this branch ✅: https://readthedocs.org/projects/tiledbsoma/builds/26331078/

Changes:
Use Ubuntu 24.04 in RTD build

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.63%. Comparing base (f5a3dbc) to head (d23f552).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3339      +/-   ##
==========================================
+ Coverage   85.53%   85.63%   +0.10%     
==========================================
  Files          54       54              
  Lines        5703     5703              
==========================================
+ Hits         4878     4884       +6     
+ Misses        825      819       -6     

Flag	Coverage Δ
python	85.63% <ø> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
python_api	85.63% <ø> (+0.10%)	⬆️
libtiledbsoma	∅ <ø> (∅)

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[johnkerl]

🚢