build(deps): bump redis from 3.5.3 to 4.3.4 in /backend

Bumps [redis](https://github.com/redis/redis-py) from 3.5.3 to 4.3.4.
- [Release notes](https://github.com/redis/redis-py/releases)
- [Changelog](https://github.com/redis/redis-py/blob/master/CHANGES)
- [Commits](redis/redis-py@3.5.3...v4.3.4)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -9,6 +9,6 @@ google-cloud-storage==2.4.0
 gunicorn==20.1.0
 python-dateutil==2.8.2
 pytoml==0.1.21
-redis==3.5.3
+redis==4.3.4
 swagger-ui-bundle==0.0.9
 zstandard==0.18.0