Redis cluster is not allowed as a valid option in config validation

**Gorse version**
Version: v0.4.14
API version: v0.2.7
Go version: go1.20.4
Git commit: ab030fd42b0aedafc7f9aacbf63df5e2b60be040
Built: Tue Jun 6 01:02:20 UTC 2023
OS/Arch: linux/amd64

**Describe the bug**
When trying to pass in `redis+cluster` the config validation fails as it is not a valid prefix

**To Reproduce**
Build a redis cluster and point the redis endpoint to `redis+cluster://URI:PORT`
Try to start gorse

**Expected behavior**
Gorse should start and properly use the redis cluster