Update storage spec for statefulset in minikube overlay

[abeatrix]

In our minikube overlays, the current patch to update the storage spec to 10Gi in the kustomization.yaml file only targets PersistentVolumeClaim (pgsql, minios etc), while gitserver and indexed-search are still requesting 200Gi of storage, which might be a lot for a minikube instance considering most of the resources are removed:

  volumeClaimTemplates:
  - metadata:
      labels:
        deploy: sourcegraph
      name: data
    spec:
      accessModes:
      - ReadWriteOnce
      resources:
        requests:
          storage: 200Gi
      storageClassName: standard

This PR adds an extra patch in the kustomization.yaml file for adjusting the storage spec for both gitserver and indexed-search, which is currently set to 200Gi respectively by default.

Also added instructions in README on how to adjust the values when needed.

Checklist

• [ ] CHANGELOG.md updated
• [ ] K8s Upgrade notes updated
• [ ] Sister deploy-sourcegraph-docker change:
• [ ] All images have a valid tag and SHA256 sum

Test plan

Manually run the generate cluster command to confirm the storage value are updated for apps_v1_statefulset_gitserver.yaml file:

...
  volumeClaimTemplates:
  - metadata:
      name: repos
    spec:
      accessModes:
      - ReadWriteOnce
      resources:
        requests:
          storage: 50Gi
      storageClassName: standard

apps_v1_statefulset_indexed-search.yaml file:

...
  volumeClaimTemplates:
  - metadata:
      labels:
        deploy: sourcegraph
      name: data
    spec:
      accessModes:
      - ReadWriteOnce
      resources:
        requests:
          storage: 25Gi
      storageClassName: standard