with next major version (4.0.0?) introduce `-Xsource:3-cross` compilation

[mr-git]

Introduction of -Xsource:3-cross causes binary incompatibility. Details:

• https://docs.scala-lang.org/scala3/guides/migration/tooling-scala2-xsource3.html
• https://github.com/scala/scala/pull/10709

Main issue is with code for case classes - Case companions no longer extend FunctionN under -Xsource:3-cross, breaking binary compatibility

With new major version, it should be possible to remove deprecations and discard scalacOptsFailOnWarn := Some(false) build setting.