Make MIN_Z_FOR_LOAD/UNLOAD the same

When unloading + preheat immediately followed by a load, the carriage is
raised first up to 20mm, then again to 50mm.

With PR #2318 it makes sense to make more space for the extra extrusion
anyway, so make them the same. This moves the carriage only once _while_
preheating, which is nice.

Firmware/Configuration.h

@@ -552,7 +552,7 @@ enum CalibrationStatus
 // Try to maintain a minimum distance from the bed even when Z is
 // unknown when doing the following operations
 #define MIN_Z_FOR_LOAD    50
-#define MIN_Z_FOR_UNLOAD  20
+#define MIN_Z_FOR_UNLOAD  50
 #define MIN_Z_FOR_PREHEAT 10
 
 #include "Configuration_adv.h"