Backported fixes

• Sync language.scala with main and backport "Add tests for context bounds migration" [#19515] (https://github.com/lampepfl/dotty/pull/19515)
• Handle default implicits to context parameters under -3.4-migration [#19512] (https://github.com/lampepfl/dotty/pull/19512)

Contributors

Thank you to all the contributors who made this release possible 🎉

According to git shortlog -sn --no-merges 3.4.0-RC2..3.4.0-RC3 these are:

     4	Paweł Marks
     1	Martin Odersky
     1	Nicolas Stucki