Updated Upstream (Bukkit/CraftBukkit/Spigot)

Upstream has released updates that appears to apply and compile correctly.
This update has not been tested by PaperMC and as with ANY update, please do your own testing

Please note that this build includes changes to meet upstreams
requirements for nullability annotations. While we aim for a level of
accuracy, these might not be 100% correct, if there are any issues,
please speak to us on discord, or open an issue on the tracker to
discuss.

Bukkit Changes:
9a6a1de3 Remove nullability annotations from enum constructors
3f0591ea SPIGOT-2540: Add nullability annotations to entire Bukkit API

CraftBukkit Changes:
8d8475fc SPIGOT-4666: Force parameter in HumanEntity#sleep
8b1588e2 Fix ExplosionPrimeEvent#setFire not working with EnderCrystals
39a287b7 Don't ignore newlines in PlayerListHeader/Footer

Spigot Changes:
cf694d87 Add nullability annotations

Spigot-API-Patches/LivingEntity-setKiller.patch

@@ -5,20 +5,11 @@ Subject: [PATCH] LivingEntity#setKiller
 
 
 diff --git a/src/main/java/org/bukkit/entity/LivingEntity.java b/src/main/java/org/bukkit/entity/LivingEntity.java
-index f67711fd..3f93b70c 100644
+index 1100e305..706d8f52 100644
 --- a/src/main/java/org/bukkit/entity/LivingEntity.java
 +++ b/src/main/java/org/bukkit/entity/LivingEntity.java
-@@ -0,0 +0,0 @@ import org.bukkit.projectiles.ProjectileSource;
- import org.bukkit.util.RayTraceResult;
- import org.bukkit.util.Vector;
- 
-+import javax.annotation.Nullable;
-+
- /**
-  * Represents a living entity, such as a monster or player
-  */
 @@ -0,0 +0,0 @@ public interface LivingEntity extends Attributable, Damageable, ProjectileSource
-      */
+     @Nullable
      public Player getKiller();
  
 +    // Paper start