qa: enforce centos9 for test

Avoids problem where rocky10 packages do not exist for squid.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>

qa/suites/fs/bugs/multifs_mdsauthcaps/tasks/0-install.yaml

@@ -1,3 +1,10 @@
+teuthology:
+  postmerge:
+    - |
+      if not (yaml.os_type == "centos" and yaml.os_version == "9.stream") then
+        -- reject kclient distro overrides
+        reject()
+      end
 meta:
 - desc: |
    install ceph/squid v19.2.2