Update badges on GitHub landing page

[drbenvincent]

• The build badge now reflects the status of main. Previously if the latest ci build failed (even if it was from developmental work in a feature branch), the badge would show as failed. That gave a misleading picture of the repo status.
• I also updated the Ruff badge. Previous it was red, and red is angry and bad.

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📚 Documentation preview 📚: https://causalpy--465.org.readthedocs.build/en/465/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.67%. Comparing base (273daa2) to head (1fc192c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #465   +/-   ##
=======================================
  Coverage   94.67%   94.67%           
=======================================
  Files          32       32           
  Lines        2196     2196           
=======================================
  Hits         2079     2079           
  Misses        117      117           

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