🔎 Confirm impact of deleting keep around refs related to merge requests

Background

In #383032, we're looking into cleaning up keep around refs for old MRs (older than 6 months). We use keep around refs to ensure that dangling commits won't get cleaned up when git GC runs. But keeping a lot of keep around refs also affects git performance.

Things to confirm

1. What happens to diff notes when keep around ref pointing to a dangling commit gets cleaned up?
2. What happens to diff versions when keep around ref pointing to a dangling commit gets cleaned up?
3. What happens to imported MRs (e.g. github to GitLab) that are old when keep around refs are deleted?
4. (Other stuff that aren't foreseen).