rados/cephadm/qa/distros: update to latest distros

rados/cephadm/qa/distros: update to latest distros

- removes ubuntu_18.04 support for podman, instead we move to focal.
- use rhel_8.3 for all rhel_8 references
- use {centos/rhel}_8 instead of {rhel/centos}_latest: to keep things
  same in master and octopus since we use: rhel_8 and centos_8 as latest
  version symlinks, which differentiated after an octopus only commit.

this was not cherry picked from master as octopus had some of the
symlinks, not in sync with master, this commit does cleanup for them,
and tries to make them similar to master.

(updates introduced change by: 126217febf023a6f561fe88dbdb87b5b2f379798)
Signed-off-by: Deepika Upadhyay <dupadhya@redhat.com>