Can an older iMac be used as a monitor for the M2 Mac Mini?
Can an older iMac be used as a monitor for the M2 Mac Mini?
iMac 21.5″, macOS 10.15
No.
Review the document: Use your iMac as a display with target display mode - Apple Support
Target display mode is a feature available with late 2009-Mid 2014 iMacs. The display Mac has to be running macOS High Sierra or older and the signal producing Mac has to be running macOS Catalina (ca. 2019) or earlier.