release-controller: Treat rhel-coreos-8 the same as `machine-os-con…

…tent`

The former is going to replace the latter.  See
https://github.com/openshift/enhancements/blob/master/enhancements/ocp-coreos-layering.md

I'm *hoping* this is somehow what's necessary to have the image
that is now built by ART included in the CI payload, so that
we can get this PR to work:

openshift/machine-config-operator#3258 (comment)

core-services/release-controller/_releases/release-ocp-4.12-ci.json

@@ -13,6 +13,7 @@
         "namespace": "origin",
         "name": "4.12",
         "excludeTags": [
+          "rhel-coreos-8",
           "machine-os-content",
           "installer",
           "installer-artifacts",