[2.0.x] Reduce HOMING_FEEDRATE_XY for Ender 3 (#12105)

Homing on the Ender 3 with HOMING_FEEDRATE_XY set to 50*60 was shown (#12074) to cause the end-stop micro-switches to bottom out, causing the carriage for both X and Y axis to physically crash into the micro switch. Reducing the homing speed prevents the switches from bottoming out.

Marlin/src/config/examples/Creality/Ender-3/Configuration.h

 #endif
 
 // Homing speeds (mm/m)
-#define HOMING_FEEDRATE_XY (50*60)
+#define HOMING_FEEDRATE_XY (20*60)
 #define HOMING_FEEDRATE_Z  (4*60)
 
 // Validate that endstops are triggered on homing moves