Fix ProtectedBranch access level validations

What

Fixes ProtectedBranch / ProtectedTag validations.

Before an access_level was required even if a group_id or user_id had been set.

I've also removed a duplicated human_access_levels definition

Why

When improving the protected branches API in https://gitlab.com/gitlab-org/gitlab-ee/issues/4131 it was discovered that access_level was being required even when it didn't make sense.

Acceptance criteria

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend