Allow adding newly required presubmits that aren't run against older branches

[irbekrm]

See https://kubernetes.slack.com/archives/CDEQJ0Q8M/p1648471110452349 for context

Currently if we add a new presubmit which is required it will become required for all PRs against all branches in the repo, including backports against older branches where the new presubmit may be irreleveant or unable to succeed.

We should fix this situation.
Perhaps it is possible to specify against which branches a job should be required by using contexts?