Fix up start, monitor baud (#20326)

Marlin/src/MarlinCore.cpp

     serial_connect_timeout = millis() + 1000UL;
     while (!MYSERIAL1 && PENDING(millis(), serial_connect_timeout)) { /*nada*/ }
   #endif
-  SERIAL_ECHO_MSG("start");
+  SERIAL_ECHOLNPGM("start");
 
   #if BOTH(HAS_TFT_LVGL_UI, MKS_WIFI_MODULE)
     mks_esp_wifi_init();

platformio.ini

 extra_scripts = ${common.extra_scripts}
 build_flags   = ${common.build_flags}
 lib_deps      = ${common.lib_deps}
-monitor_speed = 115200
+monitor_speed = 250000
 monitor_flags =
   --quiet
   --echo