FAQ for the single codebase

In https://gitlab.com/gitlab-org/gitlab-ee/issues/13304 we list the steps to take to move to a single codebase. One step to take is to close CE merge requests, and post a comment explaining why we are doing this.

This issue exists to keep track of frequently asked questions that issue and merge request authors might have. This allows us to keep the other work related issues free of long discussion threads.

For every question you might have, please create a discussion thread, not a single comment. This makes it a bit easier to see and reply to the question, instead of questions and answers getting mixed together: