Bump flask from 2.3.2 to 3.0.3 in /event-handler (#66)

Bumps [flask](https://github.com/pallets/flask) from 2.3.2 to 3.0.3.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@2.3.2...3.0.3)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

event-handler/requirements.txt

@@ -1,4 +1,4 @@
-Flask==2.3.2
+Flask==3.0.3
 gunicorn==20.1.0
 google-cloud-pubsub==2.20.1
 google-cloud-secret-manager==0.1.0