fix: examples/cloud_run_cloud_events/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14192442
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14192443

Author: snyk-bot

examples/cloud_run_cloud_events/requirements.txt

@@ -1,3 +1,4 @@
 # Optionally include additional dependencies here
 cloudevents>=1.2.0
 requests
+urllib3>=2.6.0 # not directly required, pinned by Snyk to avoid a vulnerability