Deprecation of GHC.Pack (phase 3)

In #21461 (closed) the decision has been taken to deprecate GHC.Pack.

This ticket marks the end of the three-release cycle. At this point, the GHC.Pack must have been advertised as deprecated:

1. By prose (GHC 9.6) (#21541 (closed))
2. By warning (GHC 9.8) (#21540 (closed))
3. By error warning (GHC 9.10) (we skipped it by accident)
4. By error (GHC 9.12) (for real now)