Bump flask from 2.3.3 to 3.1.3 in /end2end/server

Bumps [flask](https://github.com/pallets/flask) from 2.3.3 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@2.3.3...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]

end2end/server/requirements.txt

@@ -1 +1 @@
-Flask==2.3.3
+Flask==3.1.3