What cable/adapter do I need to attach older 21" imac to new mac minI?

Target Display Mode is only supported on 27 inch iMacs from 2009 thru 2013 running macOS High Sierra or earlier. A cable is not the issue here.

Yes, as many are with 21 inch iMacs are finding out, it does. Target Display Mode is not available in Late-2014 and more recent iMacs of any screen size either.