This refactors ServicePing::SubmitService to remove Service Ping payload
generation logic from it and let it have one responsibility - sending
the payload and handling the response from version-gitlab-com.

Since https://gitlab.com/gitlab-org/gitlab/-/merge_requests/85503,
payload generation is instead handled in GitlabServicePingWorker, where
it's then passed as an argument to SubmitService.

Relevant specs, docs and rake tasks are updated accordingly. Some specs
had to be removed in their entirety, since they tested generating the
payload rather than sending it.