Adopt towncrier for changelogs

[poljar]

This closes #5999.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.93%. Comparing base (1136eb4) to head (8cdd930).
⚠️ Report is 30 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6543      +/-   ##
==========================================
- Coverage   89.94%   89.93%   -0.01%     
==========================================
  Files         381      381              
  Lines      105753   105753              
  Branches   105753   105753              
==========================================
- Hits        95115    95114       -1     
- Misses       6993     7001       +8     
+ Partials     3645     3638       -7     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing poljar/towncrier (8cdd930) with main (d0cb664)}