Change deprecation documentation for register command
What does this MR do?
This MR follows up !3702 (merged) to implement some changes being documented in gitlab!104616 (merged).
Why was this MR needed?
To make the migration path easier, we won't deprecate the full register command, but rather only the use of registration tokens, and specifying runner parameters.
What's the best way to test this MR?
Registering a runner with a registration token (which doesn't include the upcoming glrt- prefix) should log a deprecation warning.