qa/tasks: added "is down" warning to thrash-old-clients ingorelist

This integration test failed due to an "osd down" warning,
but we intentionally mark OSDs down in this test, so the warning should
be ignored.

Fixes: https://tracker.ceph.com/issues/67820

Signed-off-by: Linjing Li <[email protected]>

Author: li-positiveandfocus

qa/tasks/thrashosds-health.yaml

@@ -30,3 +30,4 @@ overrides:
       - out of quorum
       - noscrub
       - nodeep-scrub
+      - is down