Fix Per World Difficulty / Remembering Difficulty

Fixes per world difficulty with /difficulty command and also
makes it so that the server keeps the last difficulty used instead
of restoring the server.properties every single load.

paper-server/patches/sources/net/minecraft/server/dedicated/DedicatedServer.java.patch

@@ -250,6 +250,15 @@
      @Override
      public boolean isSpawningMonsters() {
          return this.settings.getProperties().spawnMonsters && super.isSpawningMonsters();
+@@ -227,7 +337,7 @@
+ 
+     @Override
+     public void forceDifficulty() {
+-        this.setDifficulty(this.getProperties().difficulty, true);
++        // this.setDifficulty(this.getProperties().difficulty, true); // Paper - per level difficulty; Don't overwrite level.dat's difficulty, keep current
+     }
+ 
+     @Override
 @@ -286,13 +396,14 @@
          }