CI is currently broken because Bitnami has pulled all `bitnami/redis`
images.
The current official Redis image on Docker Hub is `redis`.
### Changes 🏗️
- Replace `bitnami/redis:6.2` by `redis:latest` in Backend CI workflow
file
- Make `REDIS_PASSWORD` optional in the backend settings
### Checklist 📋
#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] CI no longer broken