what OS can I upgrade to? Now running High Sierra 10.13.6 on a iMac (21.5-inch, Mid 2011)

High Sierra is the newest macOS version that your 2011 iMac can run. There is no further upgrade path.

So to run any of the newer OS versions a newer iMac would be required?

Yes, that is correct. A 2011 simply does not have the hardware (no it cannot be upgraded) to run versions of Mac OS beyond 10.13.6.