Add details of registration token deprecation to documentation
What does this MR do and why?
Describe in detail what your merge request does and why.
This MR addresses a lack of context in the authentication token documentation regarding the work being done in https://docs.gitlab.com/ee/architecture/blueprints/runner_tokens. This concern was raised by a user in #380872 (comment 1273006097).
We should also discuss whether we want to add any information to the authentication token row in https://docs.gitlab.com/ee/api/runners.html#registration-and-authentication-tokens. cc @DarrenEastman
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