Investigate listening on GCSRepository Secrets

If a user is connecting to a GCS Backup Repository, then they have to provide a secret that contains the GCS credentials as a file. We should investigate whether we need to restart the Solr Pod if this GCS credential is updated.

The `gcs-repository` implementation might re-load the credentials everytime it is used. If that's the case, then the [Secrets _should_ be auto-updated by kubernetes within the Pod](https://kubernetes.io/docs/concepts/configuration/secret/#mounted-secrets-are-updated-automatically), so we won't have to restart the Pods.

If the `gcs-repository` does not reload the credentials ever, then we will need to restart the Solr Pods to propagate the new credentials values. This can be done by setting a hash of the secret value as an annotation for the Pod. We do it for other secrets, we might just need to make the current logic more generic to accept more secrets.

Start by checking whether the gcs-repository implementation reloads credentials when used, then inspect the existing logic that hashes secrets in Pod annotations. Confirm whether Kubernetes-mounted Secret updates reach the repository without a Solr Pod restart; done means documenting the behavior and either confirming no change is needed or defining the required generic restart handling.

Written by the indexing model from the issue text.