ci: using python3.12 as main testing python version

[germa89]

Description

As the title.

Issue linked

Making a smoother transition with respect to #3605.

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 88.35%. Comparing base (dd2c3c2) to head (5782567).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3767      +/-   ##
==========================================
- Coverage   88.37%   88.35%   -0.03%     
==========================================
  Files         187      187              
  Lines       14721    14717       -4     
==========================================
- Hits        13010    13003       -7     
- Misses       1711     1714       +3     

[germa89]

To merge this before #3605

[germa89]

@pyansys-ci-bot LGTM

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬