Draft: Move towards the concept of a runner deployment token

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR updates the Runner registration token blueprint to include @josephburnett's suggestion to do away with the authentication token rather than the registration token.

Latest documentation preview link: http://main-ee-104273.docs.gitlab-review.app/ee/architecture/blueprints/runner_tokens/index.html

If this is merged, we'll also need to revise the following deliverables:

• Implementation plan in the epic

• Deprecation files introduced in %15.6:

  • Deprecation - Support for registration tokens a... (#379743 - closed)
  • Deprecation - Support for registration tokens a... (#380872 - closed)
  • Deprecation - runnerRegistrationToken in GitLab... (#381111 - closed)
  • Deprecation - runner-registration-token in GitL... (#382077 - closed)

• Deprecate POST /api/v4/verify endpoint

• GitLab Unfiltered video

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MR acceptance checklist

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Part of https://gitlab.com/gitlab-org/gitlab-runner/-/issues/28915, https://gitlab.com/gitlab-org/architecture/tasks/-/issues/27