qa/suites/rbd/valgrind: pin to centos_9.stream instead of rpm_latest

This used to be the case before commit d4b977afdc59 ("qa/distros:
rename centos_latest.yaml to rpm_latest.yaml") and subsequent changes
to enable Rocky 10.  When paired with valgrind, python_api_tests* jobs
fail persistently on Ubuntu and Rocky, see [1].  Switch back until that
is resolved (or for as long as centos_9.stream remains in the mix).

[1] https://tracker.ceph.com/issues/74864

Fixes: https://tracker.ceph.com/issues/77982
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>