`bin/sidekiq-cluster` (either directly, or via `bin/background_jobs`)
has been the only entry point for a while to launch Sidekiq processes
in:

- Omnibus.
- Helm charts.
- GDK.
- Source installs using sysvinit.

However, our systemd unit file example was still using `bundle exec
sidekiq`. Switching that to `bin/background_jobs` means that we can
finally claim that `bin/sidekiq-cluster` is the only entry point
everywhere, and simplify some of the Sidekiq system checks.

Changelog: changed