Drop extension when rolling back all PGroonga migrations

priv/repo/optional_migrations/pgroonga/20230802195431_create_pgroonga_index.exs

@@ -19,6 +19,8 @@ def up do
   def down do
     execute("DROP INDEX IF EXISTS object_content_pgroonga")
 
+    execute("DROP EXTENSION IF EXISTS pgroonga")
+
     create_if_not_exists(
       index(:objects, ["(to_tsvector('english', data->>'content'))"],
         using: :gin,