bump volumes-operator version 1.7 -> 1.8 for release 1.8

[orfeas-k]

This issue tracks the process of bumping kubeflow-volumes-operator version from 1.7 to 1.8. For the process, we 're following our internal release handbook document that has a section about manifest files upgrades and images upgrades.

The changes that this process will introduce should match the upstream ones kubeflow/manifests#2522 and they are expected to be templates updates and image versions' bumps.

[orfeas-k]

These changes introduced a configMap and a corresponding volume for which I followed what we had done recently in katib-operators described here canonical/katib-operators#122 (comment) but updated it with my current understanding. How we should implement those, in podSpec charms at least, is:

        volumeMounts: 
        - name: viewer-spec
          mountPath: /etc/config/viewer-spec.yaml
          subPath: viewer-spec.yaml 

                            {
                                "name": $name,
                                "mountPath": $mountPath - $subPath,
                                "files": [
                                    {
                                        "path": $subPath,
                                        "content": ...
                                        ),
                                    }
                                ],
                            },