Add operator release validation job

What does this MR do?

As a followup of !841 (merged)

• Changed create_followup_issue to produce a dotenv artifact containing the release issue ID
• Note: it needs an extra CatalogSource to be installed on the CI cluster Applied

apiVersion: operators.coreos.com/v1alpha1
kind: CatalogSource
metadata:
  name: operatorhub
  namespace: openshift-marketplace
spec:
  displayName: OperatorHub.io Operators
  sourceType: grpc
  image: quay.io/operatorhubio/catalog
  publisher: OperatorHub.io
  updateStrategy:
    registryPoll:
      interval: 10m

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.

Related issues

#1261 (closed)