firestore_database: Deprecate deletion_policy

Author: rwhogg

mmv1/products/firestore/Database.yaml

@@ -1,4 +1,4 @@
-# Copyright 2024 Google Inc.
+ Copyright 2024 Google Inc.
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
@@ -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'