Backported fixes

• Sync language.scala with main and backport "Add tests for context bounds migration" #19515
• Handle default implicits to context parameters under -3.4-migration #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