ci: warn about edge case on back compat tests
Created by: jhchabran
Context TL;DR, a commit was made on main, after the release cut, but before it was actually released, which included a change on the flake files for back compat tests.
When the release was actually made, the latest version to test against in the back compat test was bumped and therefore the flakes that were added were not taken in account anymore.
This is an edge case and the best solution is simply to wait for the release process to be complete.
This PR warns about this explicitly to avoid falling in the same trap.
Test plan
n/a purely textual change of CI annotation.