Set up periodics against 'previous previous' branch

We currently support n-2 releases of cert-manager, but don't only run the periodics against the previous branch, not the one before that.

We should also run them against the 'previous previous' branch

We had a discussion about this on Slack https://kubernetes.slack.com/archives/CDEQJ0Q8M/p1654700642951309?thread_ts=1654691889.270499&cid=CDEQJ0Q8M