doc: Remove SysVinit/upstart references from user-facing docs

Ceph has required systemd for several release cycles. Remove the
"Running Ceph with SysVinit" section from operating.rst and update
two other files that still listed sysvinit as a valid init system.

- rados/operations/operating.rst: remove the SysVinit section and
  the stale distribution list in the systemd intro paragraph
- ceph-volume/intro.rst: remove "(upstart, sysvinit, etc.)" from
  the ceph-disk historical context paragraph
- start/os-recommendations.rst: remove ``sysvinit`` as an example
  init framework; systemd is the only supported option

Fixes: https://tracker.ceph.com/issues/77187

Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>