chore: drop python3.9 support

[germa89]

Description

Dropping python3.9 official support.

Using python3.11 for official testing.

However, I am going to keep one outdated version (python3.9) as part of the CICD, just testing installation and imports. Nevermind, to do this I need to install the library with --ignore-requires-python argument but since we are using an ansys/action probably it is not worthy to remove completely the action just for this smoke test.

Issue linked

NA

Checklist

• I have visited and read Develop PyMAPDL.
• I have tested my changes locally
• I have added necessary documentation or updated existing documentation.
• I have followed the PyMAPDL coding style guidelines.
• I have added appropriate unit tests that also ensure the minimal coverage criteria.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have marked this PR as draft if it is not ready to be reviewed yet.
• I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.12%. Comparing base (a050034) to head (c090c08).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3326      +/-   ##
==========================================
- Coverage   87.17%   87.12%   -0.06%     
==========================================
  Files          55       55              
  Lines        9773     9773              
==========================================
- Hits         8520     8515       -5     
- Misses       1253     1258       +5