GHA to ensure milestone

[TonyCTHsu]

What

1. Remove draft notes GHA -> migrate to release script, https://github.com/DataDog/ruby-guild/pull/139
2. Remove milestone creation GHA -> migrate to release script, https://github.com/DataDog/ruby-guild/pull/140
3. Change adding milestone the pull request by guessing the milestone title starts with the major version from the gemspec of the merged branch.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.24%. Comparing base (ffc2546) to head (fabd2b2).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3570      +/-   ##
==========================================
- Coverage   98.27%   98.24%   -0.04%     
==========================================
  Files        1274     1254      -20     
  Lines       75233    74338     -895     
  Branches     3545     3527      -18     
==========================================
- Hits        73934    73030     -904     
- Misses       1299     1308       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.