Validate classification label on create & update

What does this MR do?

This validates the classification label when creating & updating a project with the external classification control enabled. It will present an error to the user and skip saving the project if the user does not have access to the newly picked classification label.

When the user tries the remove the classification label, the default one is checked.

Validation messages are the ones used from the external service if possible:

Or a default label "Access to 'authorized' not allowed" is used.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/5336