Update flask requirement from ~=2.0 to ~=2.3 in /frontend

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...2.3.3)

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

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

frontend/requirements.txt

@@ -1,4 +1,4 @@
-flask~=2.0
+flask~=2.3
 flasgger~=0.9.7
 flask_restful~=0.3.10
 elasticsearch~=7.0