Merge branch 'diff-border-radius' into 'master'

Fix border radius on MR diffs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/148716



Merged-by: Annabel Dunstone Gray <annabel.dunstone@gmail.com>
Approved-by: Julia Miocene <jmiocene@gitlab.com>
Approved-by: Stanislav Lashmanov <slashmanov@gitlab.com>