qa: use centos 9.stream for cephfs stock kernel testing

RHEL8 is no longer supported in Squid. RHEL9 is not yet available in FOG.

Fixes: https://tracker.ceph.com/issues/64085
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

diff --git a/qa/cephfs/mount/kclient/overrides/distro/stock/centos_9.stream.yaml b/qa/cephfs/mount/kclient/overrides/distro/stock/centos_9.stream.yaml
new file mode 120000 (symlink)
index 0000000..dc442c2
--- /dev/null
+++ b/qa/cephfs/mount/kclient/overrides/distro/stock/centos_9.stream.yaml
@@ -0,0 +1 @@
+.qa/distros/all/centos_9.stream.yaml
\ No newline at end of file

diff --git a/qa/cephfs/mount/kclient/overrides/distro/stock/rhel_8.yaml b/qa/cephfs/mount/kclient/overrides/distro/stock/rhel_8.yaml
deleted file mode 120000 (symlink)
index 133acf2..0000000
--- a/qa/cephfs/mount/kclient/overrides/distro/stock/rhel_8.yaml
+++ /dev/null
@@ -1 +0,0 @@
-.qa/distros/all/rhel_8.yaml
\ No newline at end of file