Content view legacy flags: change error into warning and add an explicative message

When a project in 6.0 - is migrated to 6.0 and there are some content views with flag dam or faceted search, use the warning semantic instead of error, and in the warning message be more explicit:

The flags DAM and Faceted Search are no more taken into account unless you checked one of the deprecated DAM Compat or Faceted Search target packages. The content view will be made available in the Search Tab. If you don't use deprecated packages on your 6.0, you should explicitly check the "Search content view" flag.