Make WorldCreator#keepSpawnLoaded return the WorldCreator (Fixes #8321) (#8371)

* Make WorldCreator#keepSpawnLoaded return the WorldCreator (Fixes #8321)

Thought that I fixed this before push/merge, apparently not, hence, fix
the return type and add mitigation for this.

* Fix patch/build

* Rebuild patches

patches/api/WorldCreator-keepSpawnLoaded.patch

@@ -38,10 +38,13 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +     * configuration, otherwise, will act as an override towards this setting
 +     *
 +     * @param keepSpawnLoaded the new value
++     * @return This object, for chaining
 +     */
-+    public void keepSpawnLoaded(@NotNull net.kyori.adventure.util.TriState keepSpawnLoaded) {
++    @NotNull
++    public WorldCreator keepSpawnLoaded(@NotNull net.kyori.adventure.util.TriState keepSpawnLoaded) {
 +        java.util.Objects.requireNonNull(keepSpawnLoaded, "keepSpawnLoaded");
 +        this.keepSpawnLoaded = keepSpawnLoaded;
++        return this;
 +    }
 +
 +    // Paper end

patches/server/Mitigate-effects-of-WorldCreator-keepSpawnLoaded-ret.patch

@@ -0,0 +1,25 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <theboyetronic@gmail.com>
+Date: Mon, 19 Sep 2022 00:13:02 +0100
+Subject: [PATCH] Mitigate effects of WorldCreator#keepSpawnLoaded ret type
+ change
+
+TODO: Remove in 1.21?
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/util/Commodore.java b/src/main/java/org/bukkit/craftbukkit/util/Commodore.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/main/java/org/bukkit/craftbukkit/util/Commodore.java
++++ b/src/main/java/org/bukkit/craftbukkit/util/Commodore.java
+@@ -0,0 +0,0 @@ public class Commodore
+                         {
+                             desc = getOriginalOrRewrite(desc);
+                         }
++                        if (owner.equals("org/bukkit/WorldCreator") && name.equals("keepSpawnLoaded") && desc.equals("(Lnet/kyori/adventure/util/TriState;)V")) {
++                            super.visitMethodInsn(opcode, owner, name, "(Lnet/kyori/adventure/util/TriState;)Lorg/bukkit/WorldCreator;", itf);
++                            // new method has a return, so, make sure we pop it
++                            super.visitInsn(Opcodes.POP);
++                            return;
++                        }
+                         // Paper end
+ 
+                         if ( modern )