config forking should ensure image tag is valid

[BenTheElder]

What happened:

#34651 (comment) / #34672

because we introduced the 1.33 image variants a few days ago, but haven't bumped the jobs to new images, the mutated image tag from the existing jobs to -1.33 was not available, causing jobs to fail

What you expected to happen:

We should probably detect and handle this. It might make sense to just adopt the latest available on the new branch.

How to reproduce it (as minimally and precisely as possible):
n/a

Please provide links to example occurrences, if any:

n/a

Anything else we need to know?:

Low prio, something to consider before ~4 months from now.

/sig release testing

[BenTheElder]

#34592 is the current automated image bump PR

we very intentionally don't merge these constantly, as we rarely need anything from these updates, but they carry a broad risk and may need reverting