CHORE: update CHANGELOG for v0.15.3

[pyansys-ci-bot]

Update CHANGELOG for v0.15.3 and remove .md files in doc/changelog.d.

Caution

Do not merge this pull request until the release has been successfully made.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.06%. Comparing base (292a2ad) to head (ed39c52).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5981      +/-   ##
==========================================
- Coverage   85.07%   85.06%   -0.01%     
==========================================
  Files         163      163              
  Lines       62785    62785              
==========================================
- Hits        53415    53409       -6     
- Misses       9370     9376       +6     

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