Update documentation to mention that --registration-token is deprecated
What does this MR do?
This MR updates the https://docs.gitlab.com/runner/register/#one-line-registration-command section to mention that the registration token is deprecated, and remove the command-line arguments that are no longer supported when passing an authentication token.
Why was this MR needed?
To avoid confusing users by suggesting a workflow that is now deprecated.
What's the best way to test this MR?
What are the relevant issue numbers?
Edited by Pedro Pombeiro