What cable do I use to enable my old iMac to be used as a monitor

Won't work. The new Mac mini does not support using an iMac as a display.

carefully read > Use your iMac as a display with target display mode - Apple Support

You need an external display for the new Mac mini.

see > Connect a display to Mac mini - Apple Support

Not possible unfortunately.

Nope.

the good news is a normal monitor would use a fraction of the power of the imac so you'll save on your electric bill