Broken link in `scala.language` documentaion

[nicolasstucki]

The link https://scalacenter.github.io/scala-3-migration-guide/docs/scala-3-migration-mode is broken in

• https://github.com/lampepfl/dotty/blob/master/library/src/scala/runtime/stdLibPatches/language.scala#L139
• https://github.com/lampepfl/dotty/blob/master/library/src/scala/runtime/stdLibPatches/language.scala#L146
• https://github.com/lampepfl/dotty/blob/master/library/src/scala/runtime/stdLibPatches/language.scala#L154
• https://github.com/lampepfl/dotty/blob/master/library/src/scala/runtime/stdLibPatches/language.scala#L160