qa/suites/rados/thrash-old-clients: test with N-2 releases on centos 9

qa/suites/rados/thrash-old-clients: test with N-2 releases on centos 9

It was recently decided to stop building and releasing ubuntu focal
packages for squid. This decision extended to the Shaman builds.

When we stopped building focal for squid in Shaman, this failure
started happening, because the test was looking for nonexistent
squid focal packages:
```
no results found at https://shaman.ceph.com/api/search/?project=ceph&distros=ubuntu%2F20.04%2Fx86_64&flavor=default&sha1=81127b728ce57cc8b876f0f2dd3e436633549a67
```

After a discussion in Slack, we agreed the best option going forward
would be to test on centos 9 and drop pacific from the mix, since pacific
does not have centos 9 packages. To later incorprate pacific, we will work
on a contanierized solution.

-----
Slack thread (may be expired):
https://ceph-storage.slack.com/archives/C1HFJ4VTN/p1721078395083699

Laura Flores
  4:19 PM
@Dan Mick
 I see we stopped building focal for squid on Shaman via Jenkins. I
know this is intended since we no longer plan to release squid focal
packages, but now the thrash-old-clients tests are failing on squid:
https://pulpito.ceph.com/teuthology-2024-07-14_21:00:02-rados-squid-distro-default-smithi/7801302/
These tests use an older client, i.e. reef, in a squid cluster. These
older clients go as far back as N-3 (so we test pacific, reef, and
quincy clients against a squid cluster). We need a distro that is shared
between all these releases in order to do that, which up until recently
was focal. Can we reintroduce focal shaman builds? We can put a note in
https://docs.ceph.com/en/latest/start/os-recommendations/#platforms to
explain that these packages are not released for squid, but are used to
test old clients.

Laura Flores
  4:21 PM
In the above scenario, we could consider switching to centos 9 since squid,
reef and quincy share these. But we also test against pacific clients, and
pacific of course does not build c9.

Casey Bodley
  41 minutes ago
it would be nice if those tests could eventually use containers for the upgraded
servers

Josh Durgin
  4:46 PM
centos 9 is the easiest path for now, for quincy and reef
4:46
agree with casey containerized servers would be better going forward anyway

Signed-off-by: Laura Flores <lflores@ibm.com>
(cherry picked from commit 2164873a6410e884f308209c39869137efe28878)