Fix CHAMBER_MAXTEMP security margin (#16600)

Marlin/src/lcd/menu/menu_temperature.cpp

   // Chamber:
   //
   #if HAS_HEATED_CHAMBER
-    EDIT_ITEM_FAST(int3, MSG_CHAMBER, &thermalManager.temp_chamber.target, 0, CHAMBER_MAXTEMP - 5, thermalManager.start_watching_chamber);
+    EDIT_ITEM_FAST(int3, MSG_CHAMBER, &thermalManager.temp_chamber.target, 0, CHAMBER_MAXTEMP - 10, thermalManager.start_watching_chamber);
   #endif
 
   //

Marlin/src/module/temperature.h

       static void setTargetChamber(const int16_t celsius) {
         temp_chamber.target =
           #ifdef CHAMBER_MAXTEMP
-            _MIN(celsius, CHAMBER_MAXTEMP)
+            _MIN(celsius, CHAMBER_MAXTEMP - 10)
           #else
             celsius
           #endif