Bring the scala3-v2 branch up-to-date, review, publish

[adamw]

We've started work on Magnolia v2 for Scala 3 some time ago, which included support for AnyVals some time ago, but the effort has stalled. Since then, the scala3 and scala3-v2 branches have diverged. This is a an umbrella task to fix this situation and push the v2 release:

• merge any changes from scala3 to scala3-v2
• review & merge Add isSingletonCasesEnum flag #457 (comment)
• verify that [question] Derivation differences between Scala 2 and 3 #550 (comment) is fixed with the new release, update magnolia usages in tapir
• make sure all scala 3 & scala 2 types are supported in the new release
• review the code to make sure the API & macro usage is consistent