chore(revert): revert update post processor image (#13493)

Reverts #13490

Author: ohmayr

.github/.OwlBot.lock.yaml

@@ -13,4 +13,4 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-python-mono-repo:latest
-  digest: sha256:4ba56619e605bcaa75a87f8122f285bc437ec8e23576e12a3acac3f96f1634fa
+  digest: sha256:db9568404b062e3bcf060fb7db09ca9aa51275d187b0f4ccd5f0071e82e4521a