Remove Sidekiq standalone references from systemd examples

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 (https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/5513036fc30b718da485e98bdb51c30303575976/files/gitlab-cookbooks/gitlab/templates/default/sv-sidekiq-run.erb#L13).
• Helm charts (https://gitlab.com/gitlab-org/build/CNG/-/blob/cc7c2072c509157bcd6c81ba7e3690d0d024b30e/gitlab-sidekiq/scripts/process-wrapper#L42).
• GDK (https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/d62cef019679bc45a687ed984e6330e1ec7a3b03/support/templates/Procfile.erb#L22).
• Source installs using sysvinit (https://gitlab.com/gitlab-org/gitlab/-/blob/a4e0853ec56e9c5265dc67fbb4dcd5a4ff71e90d/lib/support/init.d/gitlab#L296).

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.

For #323225 (closed) and gitlab-com/gl-infra/scalability#243 (closed).