Temporarily skip recovery deadlock test in back branches.

Temporarily skip recovery deadlock test in back branches.

The recovery deadlock test has a timing issue that was fixed in 5136967f1eb in
HEAD. Unfortunately the same fix doesn't quite work in the back branches: 1)
adjust_conf() doesn't exist, which is easy enough to work around 2) a restart
cleares the recovery conflict stats < 15.

These issues can be worked around, but given the upcoming set of minor
releases, skip the problematic test for now. The buildfarm doesn't show
failures in other parts of 031_recovery_conflict.pl.

Discussion: https://postgr.es/m/20220506155827.dfnaheq6ufylwrqf@alap3.anarazel.de
Backpatch: 10-14