3.3.0-RC6 release procedure

[Kordyjan]

• Publish artifacts to Maven via CI
  • On release-3.3.0 branch:
    • In Build.scala: Set baseVersion to 3.3.0-RC6
    • In Build.scala: Set previousDottyVersion to 3.3.0-RC5
    • in changelogs add file with a changelog
    • Tag the branch as 3.3.0-RC6
• On main:
  • In Build.scala: Set referenceVersion to 3.3.0-RC6
    Set reference version to 3.3.0-RC6 #17504
• Announce the release
  • Publish release on GitHub Releases
  • Comment on the announcement thread on https://contributors.scala-lang.org