build(deps): bump flask from 2.2.3 to 3.1.1 in /backend

Bumps [flask](https://github.com/pallets/flask) from 2.2.3 to 3.1.1.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@2.2.3...3.1.1)

---
updated-dependencies:
- dependency-name: flask
  dependency-version: 3.1.1
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/requirements.txt

@@ -1,7 +1,7 @@
 -e ../tools #egg=code-coverage-tools
 connexion==2.14.2
 datadog==0.51.0
-Flask==2.2.3
+Flask==3.1.1
 Flask-Cors==6.0.0
 flask-talisman==1.1.0
 google-cloud-storage==2.18.2