Fix compile error with 6 hot-ends (#15405)

Marlin/src/lcd/extensible_ui/ui_api.cpp

     #endif
       {
         #if HOTENDS
-          static constexpr int16_t heater_maxtemp[HOTENDS] = ARRAY_BY_HOTENDS(HEATER_0_MAXTEMP, HEATER_1_MAXTEMP, HEATER_2_MAXTEMP, HEATER_3_MAXTEMP, HEATER_4_MAXTEMP);
+          static constexpr int16_t heater_maxtemp[HOTENDS] = ARRAY_BY_HOTENDS(HEATER_0_MAXTEMP, HEATER_1_MAXTEMP, HEATER_2_MAXTEMP, HEATER_3_MAXTEMP, HEATER_4_MAXTEMP, HEATER_5_MAXTEMP);
           const int16_t e = heater - H0;
           thermalManager.setTargetHotend(constrain(value, 0, heater_maxtemp[e] - 15), e);
         #endif
 
   void setTargetTemp_celsius(float value, const extruder_t extruder) {
     #if HOTENDS
-      constexpr int16_t heater_maxtemp[HOTENDS] = ARRAY_BY_HOTENDS(HEATER_0_MAXTEMP, HEATER_1_MAXTEMP, HEATER_2_MAXTEMP, HEATER_3_MAXTEMP, HEATER_4_MAXTEMP);
+      constexpr int16_t heater_maxtemp[HOTENDS] = ARRAY_BY_HOTENDS(HEATER_0_MAXTEMP, HEATER_1_MAXTEMP, HEATER_2_MAXTEMP, HEATER_3_MAXTEMP, HEATER_4_MAXTEMP, HEATER_5_MAXTEMP);
       const int16_t e = extruder - E0;
       enableHeater(extruder);
       thermalManager.setTargetHotend(constrain(value, 0, heater_maxtemp[e] - 15), e);

Marlin/src/lcd/menu/menu.h

 extern bool screen_changed;
 
 #if HOTENDS
-  constexpr int16_t heater_maxtemp[HOTENDS] = ARRAY_BY_HOTENDS(HEATER_0_MAXTEMP, HEATER_1_MAXTEMP, HEATER_2_MAXTEMP, HEATER_3_MAXTEMP, HEATER_4_MAXTEMP);
+  constexpr int16_t heater_maxtemp[HOTENDS] = ARRAY_BY_HOTENDS(HEATER_0_MAXTEMP, HEATER_1_MAXTEMP, HEATER_2_MAXTEMP, HEATER_3_MAXTEMP, HEATER_4_MAXTEMP, HEATER_5_MAXTEMP);
 #endif
 
 void scroll_screen(const uint8_t limit, const bool is_menu);