Update flask requirement from <3,>=2.0 to >=2.0,<4 (#5157)

Updates the requirements on [flask](https://github.com/pallets/flask) to permit the latest version.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@2.0.0...3.1.0)

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

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requirements-server.txt

@@ -1,5 +1,5 @@
 connexion[swagger-ui]>=2.10.0, <3
-flask>=2.0,<3
+flask>=2.0,<4
 werkzeug>=2.0,<4
 flask-cors==5.0.0
 gunicorn==23.0.0