fix: appengine/standard/migration/urlfetch/requests/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-7267250

Author: snyk-bot

appengine/standard/migration/urlfetch/requests/requirements.txt

@@ -2,3 +2,4 @@ Flask==3.1.3; python_version >= '3.9'
 Werkzeug==3.1.8; python_version >= '3.9'
 requests==2.34.2; python_version >= '3.10'
 requests-toolbelt==1.0.0
+urllib3>=2.2.2 # not directly required, pinned by Snyk to avoid a vulnerability