chore: keep context bounds as implicits in Scala 2 mode

[hamzaremmal]

During the transition, we will keep desugaring context bounds in Scala 2 files as implicit clauses, not using clauses.

Related to #21257

[hamzaremmal]

The more I think about this change, the more I don't think it's a good idea.

To explain more: if we merge this, we offer a migration path as these context bounds will still be an implicit and we will have at call site the warning "implicit will now require using keyword". We can only revert them back to contextual parameters in 3.10.0. Also, this will be breaking the specification for these set of files (but really, we are already breaking the specification for these sets of files). If we don't merge this, people will be forced to add at call site the using keyword if required. We will still follow the specification (at least for this part) and we will not have a weird behavior where part of the context bounds in the stdlib are givens and another part are implicits and we will have what people will expect now since 3.6.0 and not be flabbergasted by yet another exception to following the specification (which this time is user facing).