Repurposing an old iMac with Ubuntu

Back in 2011, my main work machine was a 21.5-inch iMac (Mid-2011), which was an amazing piece of hardware. At that time, most of my work was done on Linux, but I switched to Mac OS X, which provided me with a beautiful graphical user interface and a terminal. High Sierra (10.13) was the last version of macOS officially supported by this iMac. I attempted to unofficially install a more recent version of macOS, but encountered…