fix: app_engine_flex/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-GUNICORN-6615672
- https://snyk.io/vuln/SNYK-PYTHON-KERAS-6615954

app_engine_flex/requirements.txt

@@ -2,4 +2,5 @@ tensorflow==2.6.0
 textgenrnn==2.0.0
 ru-proverbs==1.1.0
 flask==2.0.1
-gunicorn==20.1.0
+gunicorn==22.0.0
+keras>=2.13.1rc0 # not directly required, pinned by Snyk to avoid a vulnerability