Override cluster selector on registry GC dashboard

This is a followup from !3958 (comment 696598371).

gstg-gitlab-gke is the default selected cluster on the registry dashboards. This comes with two downsides:

1. This cluster isn't meaningful for the registry, so it's a bad default;
2. There is no way to see the aggregated metrics across the remaining (zonal) clusters.

This MR overrides the cluster selector by allowing selecting multiple clusters and defaulting to all of them.

Before:

https://dashboards.gitlab.net/d/registry-gc/registry-garbage-collection-detail?orgId=1&var-PROMETHEUS_DS=Global&var-environment=gstg&var-cluster=gstg-gitlab-gke&var-stage=main&var-namespace=gitlab&var-Deployment=gitlab-registry

After:

https://dashboards.gitlab.net/dashboard/snapshot/KQMfUaeOhYxQ4lmYfCG7aV5wVbn8F5fU?orgId=1&var-PROMETHEUS_DS=Global&var-environment=gstg&var-stage=main&var-namespace=gitlab&var-Deployment=gitlab-registry&var-cluster=All