Updates the readiness template

John Jarvisrequested to merge
jarv/readiness-update into master

Part of https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15459, this refactors our readiness review process so that it follows a checklist driven issue and expands readiness to all significant changes that will be made on gitlab.com.

This also removes the service_readiness template, instead we will use a single template for all changes.

Dependencies

Edited by John Jarvis

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