fix: license headers

[clatapie]

Description

This PR is to update the license headers to a valid format.
The argument start-year needed to be set to 2016 as it corresponds to the starting year of the project.

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 87.14%. Comparing base (e8f424d) to head (4b4171c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3307   +/-   ##
=======================================
  Coverage   87.14%   87.14%           
=======================================
  Files          55       55           
  Lines        9768     9768           
=======================================
  Hits         8512     8512           
  Misses       1256     1256           

[germa89]

LGTM. Thank you!