Bump flask from 3.0.0 to 3.1.3 in /code/default/gae_proxy/server/gae

Bumps [flask](https://github.com/pallets/flask) from 3.0.0 to 3.1.3.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@3.0.0...3.1.3)

---
updated-dependencies:
- dependency-name: flask
  dependency-version: 3.1.3
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

code/default/gae_proxy/server/gae/requirements.txt

@@ -1,4 +1,4 @@
-Flask==3.0.0
+Flask==3.1.3
 requests
 gunicorn
 google-cloud-storage