Temporarily skip recovery deadlock test in back branches.

The recovery deadlock test has a timing issue that was fixed in 5136967 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

Author: anarazel

src/test/recovery/t/031_recovery_conflict.pl

@@ -228,6 +228,10 @@
 
 
 ## RECOVERY CONFLICT 5: Deadlock
+SKIP:
+{
+	skip "disabled until after minor releases, due to instability";
+
 $sect = "startup deadlock";
 $expected_conflicts++;
 
@@ -286,6 +290,7 @@
 
 # clean up for next tests
 $node_primary->safe_psql($test_db, qq[ROLLBACK PREPARED 'lock';]);
+}
 
 
 # Check that expected number of conflicts show in pg_stat_database. Needs to