build(deps): bump connexion from 2.7.0 to 2.9.0 in /backend

Bumps [connexion](https://github.com/zalando/connexion) from 2.7.0 to 2.9.0.
- [Release notes](https://github.com/zalando/connexion/releases)
- [Commits](spec-first/connexion@2.7.0...2.9.0)

---
updated-dependencies:
- dependency-name: connexion
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -1,5 +1,5 @@
 -e ../tools #egg=code-coverage-tools
-connexion==2.7.0
+connexion==2.9.0
 datadog==0.41.0
 fakeredis==1.5.0
 Flask==1.1.2