Merge branch 'slashmanov/migrate-legacy-diffs-to-pinia' into 'master'

Migrate legacy diffs to Pinia

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163827



Merged-by: Phil Hughes <me@iamphill.com>
Approved-by: Thomas Randolph <trandolph@gitlab.com>
Approved-by: Phil Hughes <me@iamphill.com>
Co-authored-by: Stanislav Lashmanov <slashmanov@gitlab.com>