Symfony 7.4 is the new LTS; bump every symfony/* floor to ^7.4 (|| ^8.0)
across all components and drop 6.4 / 7.0-7.3 support. Also bump the
extra.symfony.require declaration in each composer.json to ^7.4 || ^8.0
to stay consistent with the new floor (read by flex in CI).

Cleanups the floor unblocks:
- ParameterValidatorProvider: drop the getConstraint()/getCause()
  method_exists shims (guaranteed on ConstraintViolationInterface >= 7.2)
- OperationRequestInitiatorTrait: drop a stale TODO