PFW-1362 Use PROGMEM messages

The main purpose of these two messages is just to see if SpoolJoin is enabled or not.
"On" and "Off" are descriptive enough.

Saves 16 bytes of flash

Firmware/SpoolJoin.cpp

@@ -1,6 +1,8 @@
 #include "SpoolJoin.h"
 #include "Marlin.h"
 #include "eeprom.h"
+#include "messages.h"
+#include "language.h"
 
 namespace SpoolJoin {
 
@@ -33,9 +35,9 @@ void SpoolJoin::initSpoolJoinStatus()
     SERIAL_ECHOPGM("SpoolJoin is ");
     if (isSpoolJoinEnabled())
     {
-        SERIAL_ECHOLNPGM("enabled");
+        SERIAL_ECHOLNRPGM(_O(MSG_ON));
     } else {
-        SERIAL_ECHOLNPGM("disabled");
+        SERIAL_ECHOLNRPGM(_O(MSG_OFF));
     }
 }