UPDATING.md entry about dashboard v2 forced migration in 0.28
UPDATING.md entry about dashboard v2 forced migration in 0.28

airbnb/caravel,airbnb/caravel,airbnb/superset,airbnb/superset,mistercrunch/panoramix,zhouyao1994/incubator-superset,airbnb/superset,airbnb/superset,apache/incubator-superset,zhouyao1994/incubator-superset,airbnb/caravel,apache/incubator-superset,mistercrunch/panoramix,mistercrunch/panoramix,zhouyao1994/incubator-superset,apache/incubator-superset,apache/incubator-superset,mistercrunch/panoramix,apache/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,airbnb/caravel