♻️(backend) avoid repeating 'service' in python modules

These modules are already stored under the 'service' folder, it was redundant.
Renamed these files based on @lunika feedbacks.

src/backend/core/authentication/backends.py

@@ -10,7 +10,7 @@ from mozilla_django_oidc.auth import (
 )
 
 from core.models import User
-from core.services.marketing_service import (
+from core.services.marketing import (
     ContactCreationError,
     ContactData,
     get_marketing_service,