doc: make instructions to get an updated cephadm common

doc: make instructions to get an updated cephadm common

As discussed in person and over the ceph orch weekly, we want all users
to use a recent supported version of cephadm. Previously, the
instructions only had those downloading cephadm with curl using the
"add-repo" and "install" commands to get a up-to-date cephadm build.
According to ADK we've seen cases of users get "old" distro packages
in the past. Change the instructions so that the "update cephadm" steps
are common after acquiring a "bootstrap copy" of cephadm.

Note: This commit is a cherry-pick of
d7921e88d69b4bc355da9c0327cc33e59e7d7abb, which was introduced by John
Mulligan in https://github.com/ceph/ceph/pull/48180. This is one of
three commits introduced in that PR, and this cherry-pick cleans up
omissions I (Zac Dover) inadvertently introduced while attempting to
rectify the merge conflicts in https://github.com/ceph/ceph/pull/51843.
I expect that two more cherry-picked commits will follow this one.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit d7921e88d69b4bc355da9c0327cc33e59e7d7abb)