Failures in "qa:mixed-cluster:v7.8.0#mixedClusterTest" tests

[danhermann]

We've had several failures from the :qa:mixed-cluster:v7.8.0#mixedClusterTest in a short period. Two examples are:

https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob+fast+bwc/4543/console
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob+fast+bwc/4542/console

[dnhatn]

@stu-elastic I think this is related to #54008. It wasn't backported but the wire version is 7.8.0 already. We need to backport that PR to 7.x or revert the change on master.

[original-brownbear]

I opened #54321 to fix the version constant for now to get BwC tests unblocked.

[stu-elastic]

Thanks, I'm backporting now.