Bump flask from 3.0.3 to 3.1.3 in /docker/oidc-test-client

Bumps [flask](https://github.com/pallets/flask) from 3.0.3 to 3.1.3.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](https://github.com/pallets/flask/compare/3.0.3...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>

docker/oidc-test-client/requirements.txt

@@ -1,2 +1,2 @@
-Flask==3.0.3
+Flask==3.1.3
 oic==1.7.0