Update Valkey Helm values.yaml image.registry from docker.io to ghcr.io

Consistent image repository for all helm charts in the valkey-helm project - please update values.yaml from "docker.io/valkey/valkey" to "ghcr.io/valkey-io/valkey"

```
image:
  # Image registry
  registry: "ghcr.io"
  # Operator container image repository
  repository: valkey-io/valkey
```