firestore_database: Deprecate deletion_policy (GoogleCloudPlatform#13936)

Author: rwhogg

mmv1/products/firestore/Database.yaml

@@ -145,6 +145,7 @@ virtual_fields:
       See also `delete_protection`.
     type: String
     default_value: "ABANDON"
+    deprecation_message: '`deletion_policy` is deprecated and will be removed in a future major release. Use `delete_protection_state` instead.'
 parameters:
 properties:
   - name: 'name'