Limit hotend / bed temperature to maxtemp-15 (#12713)

Marlin/src/module/temperature.h

       #if ENABLED(AUTO_POWER_CONTROL)
         powerManager.power_on();
       #endif
-      target_temperature[HOTEND_INDEX] = MIN(celsius, maxttemp[HOTEND_INDEX]);
+      target_temperature[HOTEND_INDEX] = MIN(celsius, maxttemp[HOTEND_INDEX] - 15);
       #if WATCH_HOTENDS
         start_watching_heater(HOTEND_INDEX);
       #endif
         #endif
         target_temperature_bed =
           #ifdef BED_MAXTEMP
-            MIN(celsius, BED_MAXTEMP)
+            MIN(celsius, BED_MAXTEMP - 15)
           #else
             celsius
           #endif