ci: pin ubuntu OS to 22.04.

[germa89]

Description

As the title.

Issue linked

Close #3658

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 (86e535c) to head (b9902c2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3659      +/-   ##
==========================================
+ Coverage   87.11%   87.12%   +0.01%     
==========================================
  Files         187      187              
  Lines       14694    14694              
==========================================
+ Hits        12800    12802       +2     
+ Misses       1894     1892       -2     

[germa89]

@pyansys-ci-bot LGTM

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬