[AAP-73644 ] Fix adjust_roles to use update_or_create for LOCKED_ROLES

[jerabekjiri]

Fixes: AAP-73644

Hub pulpcore-manager migrate fails with sycopg.errors.UniqueViolation: duplicate key value violates unique constraint "core_role_name_key" when locked roles already exist in the database with locked=False. This happens because get_or_create looks up by both name and locked=True, misses the existing role, and then tries to insert a duplicate.

How to reproduce locally

1. Start a clean Hub environment and run migrations so all roles are created normally
2. pulpcore-manager shell_plus
3. Set the role's locked flag to False to simulate the race condition:

role = Role.objects.get(name="ansible.ansiblerepository_creator")
role.locked = False
role.save()

4. pulpcore-manager migrate
5. It crashes with the UniqueViolation above because get_or_create(name=..., locked=True) doesn't find
   the existing role (it has locked=False) and tries to insert a duplicate. The crash happens in pulpcore/app/apps.py during the post_migrate signal when adjust_roles runs.

Fix

Switched to update_or_create with lookup by name only so that existing roles get corrected locked=True instead of crashing.

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough