3.2.1-RC3 Release procedure

[Kordyjan]

• Merge relevant backports:
  • Backport "Be still more careful when computing denotations of class parameters" #16196
  • Backport "Fix tuple casting" #16197
• Publish artifacts to Maven via CI
  • On release-3.2.1 branch:
    • In Build.scala: Set baseVersion to 3.2.1-RC3
    • In Build.scala: Set previousDottyVersion to 3.2.1-RC2
    • in changelogs add file with a changelog
    • Tag the branch as 3.2.1-RC3
• Announce the release
  • Publish release on GitHub Releases
  • Comment on the announcement thread on https://contributors.scala-lang.org