Automatically unregister deleted Runners
Description
Problem
Deleting a Runner via UI or API removes the Runner from the GitLab database, but the Runner configuration on the host is not updated. Users are required to manually unregister a deleted Runner. Currently, our documentation for deleting a Runner does not advise users to unregister the deleted Runner: https://docs.gitlab.com/ee/ci/runners/runners_scope.html#delete-a-project-runner
A Runner host will continue to contact GitLab while the specific Runner is still present in the configuration file. This API traffic to GitLab is unnecessary, and in some scenarios, contribute to per-IP rate limits for customers.
Proposal
Allow GitLab to notify that the Runner is deleted in response to POST /api/:version/runners/verify requests, and allow Runner to remove that Runner to unregister the Runner automatically.