drop: remove access to `Predef.any2stringadd`

[hamzaremmal]

Disclaimer: This PR might be controversial and we will probably have to debate it and set precedence on how to handle such changes.

In this PR, we break source compatibility (but not backwards binary compatibility) by removing access to the Predef.any2stringadd implicit class. It was deprecated in Scala 2.13, not imported by default in Scala 3 (behaviour that was documented nowhere). As such, and as part of cleaning up the stdlib, we drop access to it while keeping it public in the bytecode. I have also removed the implicit keyword since I've undone the logic in the compiler that removes it from the root import and it might be available scala package and sub-packages (which goes against the point).

Closes #8830