The effect needs to be compiled with the new version of MGFXC, which is in your MonoGame source folder under Tools/MonoGame.Effect.Compiler/
Just build the entire tools solution, it includes MGFXC.
You can then launch the newly built MGCB editor and build the effects manually, or, make sure to reference the newly built content pipeline properly for automatic building.
This thread contains some info on how to do the latter.