chore: exclude requirements.txt file from renovate-bot (#1594) (#811)

Source-Link: googleapis/synthtool@f58d313
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:f14e3fefe8e361e85752bd9890c8e56f2fe25f1e89cbb9597e4e3c7a429203a3

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,4 +13,4 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-java:latest
-  digest: sha256:6566dc8226f20993af18e5a4e7a2b1ba85a292b02dedb6a1634cf10e1b418fa5
+  digest: sha256:f14e3fefe8e361e85752bd9890c8e56f2fe25f1e89cbb9597e4e3c7a429203a3