qa/tests: use bionic only for old clients in rados/thrash-old-clients

Fixes: https://tracker.ceph.com/issues/46529
Signed-off-by: Yuri Weinstein <yweinste@redhat.com>
(cherry picked from commit 7e28443779f0a5c59672bf5f591e530919a8f2f9)
Signed-off-by: Yuri Weinstein <yweinste@redhat.com>

diff --git a/qa/suites/rados/thrash-old-clients/distro$/centos_7.6.yaml b/qa/suites/rados/thrash-old-clients/distro$/centos_7.6.yaml
deleted file mode 120000 (symlink)
index f406338..0000000
--- a/qa/suites/rados/thrash-old-clients/distro$/centos_7.6.yaml
+++ /dev/null
@@ -1 +0,0 @@
-.qa/distros/all/centos_7.6.yaml
\ No newline at end of file

diff --git a/qa/suites/rados/thrash-old-clients/distro$/ubuntu_18.04.yaml b/qa/suites/rados/thrash-old-clients/distro$/ubuntu_18.04.yaml
new file mode 120000 (symlink)
index 0000000..cfb85f1
--- /dev/null
+++ b/qa/suites/rados/thrash-old-clients/distro$/ubuntu_18.04.yaml
@@ -0,0 +1 @@
+.qa/distros/all/ubuntu_18.04.yaml
\ No newline at end of file