Nextcloud notes, but the setup isn't obvious:
- Create a list with user A
- Share the list .md file from the Notes folder via regular Nextcloud Files to user B
- As user B, move the shared note file to the Notes folder in the Files app
And you have a shared note. The Notes folder refers to the folder containing the user's note files, not sure what the English name is.
Not sure how well concurrency is handled, as Notes doesn't really support sharing, but if it's for groceries, I doubt it's a frequent issue. Has been working well for 2 years now.