Convert to reduced list of known types (#50010)

lib/ansible/modules/network/junos/junos_system.py

@@ -96,7 +96,7 @@ RETURN = """
 diff.prepared:
   description: Configuration difference before and after applying change.
   returned: when configuration is changed and diff option is enabled.
-  type: string
+  type: str
   sample: >
           [edit system]
           +  host-name test;