chore: Migrate changelog and news fragments to Markdown

[achimnol]

• Migrate changelog and news fragments to Mardkwon for alignment with GitHub release pages and AI tools
• Remove mis-added files

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the changes folder
  • name it <issue_id>.<type> for example (588.bug)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.05%. Comparing base (53cb286) to head (3c88ee2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #994   +/-   ##
=======================================
  Coverage   73.05%   73.05%           
=======================================
  Files          25       25           
  Lines        1837     1837           
  Branches      250      250           
=======================================
  Hits         1342     1342           
  Misses        410      410           
  Partials       85       85           

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