Disable runner registration tokens on self-managed instances

⚠ This MR is based on the pedropombeiro/413956/1-default-away-from-runner-registration-token branch, please change to master before merging ⚠

What does this MR do and why?

This MR disables runner registration tokens on self-managed instances at the instance level. The goal is to nudge admins to acknowledge the upcoming changes before we remove runner registration tokens altogether in %18.0.

Changelog: changed

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