Upgrade redis chart to new version

We didn't upgrade redis in ~1,5y and are now facing 3 major version upgrade:

-    version: 10.5.7                                                                        
+    version: 15.4.2

See https://github.com/bitnami/charts/tree/master/bitnami/redis#to-1400 with a summary of breaking changes.

Imo we should do this ASAP before we get more users.