Define G21 as NOOP w/o INCH_MODE_SUPPORT (#13229)

Ticket #13228

If `INCH_MODE_SUPPORT` is undefined, G20 is an unknown command as it should be (Marlin is, by default, operating in metric mode). G21, however, is found in many slicers and printer start gcode sections and should be accepted (as a NOOP) to avoid the unknown commands.

Marlin/src/gcode/gcode.cpp

       #if ENABLED(INCH_MODE_SUPPORT)
         case 20: G20(); break;                                    // G20: Inch Mode
         case 21: G21(); break;                                    // G21: MM Mode
+      #else
+        case 21: NOOP; break;                                     // No error on unknown G21
       #endif
 
       #if ENABLED(G26_MESH_VALIDATION)