Fix MySQL gateway migration ordering

Split replacement-key recreation so MySQL drops the old constraint before validating its reused name.

src/main/resources/db/migration/V6__gateway_execution_state.sql

@@ -59,7 +59,9 @@ ALTER TABLE usage_records
 -- A rotated refresh row points to its replacement. RESTRICT can block the
 -- account -> grant -> refresh cascade when an account is deleted.
 ALTER TABLE gateway_refresh_tokens
-    DROP FOREIGN KEY fk_gateway_refresh_replacement,
+    DROP FOREIGN KEY fk_gateway_refresh_replacement;
+
+ALTER TABLE gateway_refresh_tokens
     ADD CONSTRAINT fk_gateway_refresh_replacement
         FOREIGN KEY (replaced_by_id) REFERENCES gateway_refresh_tokens (id)
         ON DELETE SET NULL;