[2.0.x] Limit PID autotune target to maxtemp-15 (#12691)

Marlin/src/module/temperature.cpp

       next_auto_fan_check_ms = next_temp_ms + 2500UL;
     #endif
 
+    if (target > GHV(BED_MAXTEMP, maxttemp[heater]) - 15)) {
+      SERIAL_ECHOLNPGM(MSG_PID_TEMP_TOO_HIGH);
+      return;
+    }
+
     SERIAL_ECHOLNPGM(MSG_PID_AUTOTUNE_START);
 
     disable_all_heaters();