Follow-up for #3591638: Drop `image` field type's added `src_with_alternate_widths` field property, keep only `src`, provide update path
The following discussion from #3591638+'s !1217 should be addressed:
- [ ] @wimleers started a [discussion](https://git.drupalcode.org/project/canvas/-/merge_requests/1217#note_1079862): (+6 comments)
- [ ] update path to consolidate to only `src`; necessary because https://git.drupalcode.org/project/canvas/-/merge_requests/1217#note_1080686
- [ ] this means `src_with_alternate_widths` must not appear ANYWHERE in the codebase anymore, except in update path logic+fixtures
issue