Garbage collect grafana-dashboard-etcd

This is a clone of issue OCPBUGS-8044. The following is the description of the original issue:
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Description of problem:

rhbz#2089199, backported to 4.11.5, shifted the etcd Grafana dashboard from the monitoring operator to the etcd operator. During the shift, the ConfigMap was renamed from grafana-dashboard-etcd to etcd-dashboard. However, we did not include logic for garbage-collecting the obsolete dasboard, so clusters that update from 4.11.1 and similar into 4.11.>=5 or 4.12+ currently end up with both the obsolete and new ConfigMaps. We should grow code to remove the obsolete ConfigMap.

Version-Release number of selected component (if applicable):

4.11.>=5 and 4.12+ are currently exposed.

How reproducible:

100%

Steps to Reproduce:

1. Install 4.11.1.
2. Update to a release that defines the etcd-dashboard ConfigMap.
3. Check for etcd dashboards with oc -n openshift-config-managed get configmaps | grep etcd.

Actual results:

Both etcd-dashboard and grafana-dashboard-etcd exist:

$ oc -n openshift-config-managed get configmaps | grep etcd
etcd-dashboard                                        1      196d
grafana-dashboard-etcd                                1      2y282d

Another example is 4.11.1 to 4.11.5 CI:

$ curl -s https://gcsweb-ci.apps.ci.l2s4.p1.openshiftapps.com/gcs/origin-ci-test/logs/release-openshift-origin-installer-e2e-aws-upgrade/1570415394001260544/artifacts/e2e-aws-upgrade/configmaps.json | jq -r '.items[].metadata | select(.namespace == "openshift-config-managed" and (.name | contains("etcd"))) | .name'
etcd-dashboard
grafana-dashboard-etcd

Expected results:

Only etcd-dashboard still exists.

Additional info:

A new manifest for the outgoing ConfigMap that sets the release.openshift.io/delete: "true" annotation would ask the cluster-version operator to reap the obsolete ConfigMap.