Deprecate register command
What does this MR do?
This MR adds notices to the gilab-runner register command and supporting documentation to make the deprecation public.
Why was this MR needed?
We're removing the gitlab-runner register command in %16.0 as per https://docs.gitlab.com/ee/architecture/blueprints/runner_tokens/
What's the best way to test this MR?
What are the relevant issue numbers?
Edited by Pedro Pombeiro