Fully deprecate legacy sidekiq workers for onboarding

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167211



Merged-by: Doug Stull <dstull@gitlab.com>
Approved-by: Max Orefice <morefice@gitlab.com>
Approved-by: Aishwarya Subramanian <asubramanian@gitlab.com>
Approved-by: Hitesh Raghuvanshi <hraghuvanshi@gitlab.com>
Approved-by: Omar Qunsul <oqunsul@gitlab.com>
Reviewed-by: Max Orefice <morefice@gitlab.com>