Rewrote pet back end to use a more authentic system

Pets can now stack
Fixes a bug where a pet could get reset to 0 hunger and growth when dropped
Player save version migration messages are no longer shown
Pets now morph into adults in-place

Server/src/main/core/ServerConstants.kt

@@ -18,7 +18,7 @@ class ServerConstants {
         var NOAUTH_DEFAULT_ADMIN: Boolean = true
 
         @JvmField
-        var CURRENT_SAVEFILE_VERSION = 1
+        var CURRENT_SAVEFILE_VERSION = 2
 
         @JvmField
         var DAILY_ACCOUNT_LIMIT = 3