Convert Constraint plugin discovery to attribute for MediaMappingsConstraint

Reviewed the MR and changes looks good, I guess this can be mergeable ~~ Hence marking this one RTBC.

This needs a change record. See https://www.drupal.org/node/3229001 → for an example.

@quietone how can I add a change record?

I can not see the link "add change notice" in the issue sidebar.

Thanks

I've confirmed your user profil, now you can see the CR shared via @quietone ~~ I guess you need to add only this issue url over there.
@quietone please confirm on the same?

@ankitv18 Thanks!

@quietone I added a new change record (draft state).