Saturation point for zoekt nodes: kube_persistent_volume_claim_disk_space

Part of https://gitlab.com/gitlab-com/gl-infra/capacity-planning-trackers/gitlab-com/-/issues/1701. Addresses the discussion https://gitlab.com/gitlab-com/gl-infra/capacity-planning-trackers/gitlab-com/-/issues/1701#note_1901086647.

This will allow tweaking different SLOs for the capacity warnings for the zoekt nodes for the saturation point kube_persistent_volume_claim_disk_space.

The next step would be to add a service definition for zoekt gitlab-com/gl-infra/scalability#3559 and remove the custom saturation point.