Fix/improve backoff feedrates (#15300)

Marlin/src/module/motion.cpp

     ];
     if (backoff_mm) {
       current_position[axis] -= ABS(backoff_mm) * axis_home_dir;
-      line_to_current_position(Z_PROBE_SPEED_FAST);
+      line_to_current_position(
+        #if HOMING_Z_WITH_PROBE
+          (axis == Z_AXIS) ? MMM_TO_MMS(Z_PROBE_SPEED_FAST) :
+        #endif
+        homing_feedrate(axis)
+      );
     }
   #endif