Add GitLab Operator to the release tooling

What does this MR do?

Make release-tools handle the release of GitLab Operator

Notes

1. GitLab Operator, by design, is supposed to support last three stable releases of GitLab Helm Charts.
2. However, release-tools has knowledge of only the current version being worked on.
3. This means, while doing security backports, we can't be sure what the latest 3 stable versions of Charts are.
   1. Scenario: releasing security backports 14.8.9, 14.9.9 and 14.10.9
   2. There's no guarantee releases are done in that order.
   3. If 14.10.9 is released first, it will incorrectly find 14.8.8 and 14.9.8 as the two previous latest stable versions, while technically 14.8.9 and 14.9.9 will be released in a few minutes.
4. Hence, tagging of Operator can be done only after we are sure all Charts versions are tagged.
5. This means Operator can be tagged only during the Publish phase of other components.

Closes: gitlab-org/cloud-native/gitlab-operator#224 (closed)

Author Check-list

• [-] Has documentation been updated?