debug: use levels 1-5 in WIREPLUMBER_DEBUG

This makes WIREPLUMBER_DEBUG behave just like PIPEWIRE_DEBUG.
Same levels, same syntax, easier for users.

ERROR & CRITICAL are always printed, as they should; there is no point
in disabling them, since:
- ERRORs are always fatal and should never ever happen
- CRITICALs are assertion failures and indicate bugs that need to be fixed