If an upgraded node is restarted multiple times without flushing a new
index commit, then we will wrongly exclude all commits from the starting
commits. This bug is reproducible with these minimal steps: (1) create
an empty index on 6.1.4 with translog retention disabled, (2) upgrade
the cluster to 7.7.0, (3) restart the upgraded the cluster. The problem
is that with the new translog policy can trim translog without having a
new index commit, while the existing commit still refers to the previous
translog generation.
Closes#57091
5b08eaf90c