CI - drop support for older versions oc v3.6.1, v3.7.1

[dipak-pawar]

Currently we are running CI build against v3.6.1, v3.7.1, v3.7.2, v3.9.0-alpha.3. As we have #1025 issue around, we should drop support for v3.7.1 and v3.6.1

After dropping above two versions we can run build against only two versions by running one category on one node(4 node * 1 category on each node) which will reduce the number of pods (< 20) for entire build and we'll get green build for v3.7.2.

• v3.7.2
  • k8s
  • openshift
• v3.9.0-alpha.3
  • k8s
  • openshift