Updated Upstream (Bukkit/CraftBukkit)

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

Bukkit Changes:
304e83eb PR-1002: Improve documentation and implementation of getMaxStackSize
e8215ea2 SPIGOT-7638: Library loader does not seem to resolve every dependency
79c595c0 SPIGOT-7637: Bad logic in checking nullability of AttributeModifier slots

CraftBukkit Changes:
91b1fc3f1 SPIGOT-7644: Fix ItemMeta#getAsString
4e77a81e1 SPIGOT-7615: PlayerLeashEntityEvent cancelled eats lead
996f660f3 Do not remove leash knot if leasing to an existing leash knot gets cancelled
f70367d42 SPIGOT-7643: Fix inverted leash event cancelled usage and remove leash knot if no entity gets leashed
7ddb48294 SPIGOT-7640: Abnormal jumping height of wind charge
080c8711e SPIGOT-7639: Incoming plugin channels not working
ad549847e Open a direct connection instead of pinging mojang server to check if it is reachable
38e2926c5 SPIGOT-7365: DamageCause blocked by shield should trigger invulnerableTime

patches/api/0006-Adventure.patch

@@ -4323,7 +4323,7 @@ index 60ebfab9596f26cfd2d75a43f3f88f2dd56575f3..dbaf54018a7dd392378869a5a302a880
 +    // Paper end - Adventure
  }
 diff --git a/src/main/java/org/bukkit/inventory/ItemStack.java b/src/main/java/org/bukkit/inventory/ItemStack.java
-index cf1733bc76d1e29ad0f533f6e49818f83e8e3358..f2a6f62f0344684668febc0999b81748c6737826 100644
+index 257b5db1aa58d57cfbf5bd1f17f9df6c509bef90..53cc84d1ef6e281e8637ec7406236e1185ad7d82 100644
 --- a/src/main/java/org/bukkit/inventory/ItemStack.java
 +++ b/src/main/java/org/bukkit/inventory/ItemStack.java
 @@ -23,7 +23,7 @@ import org.jetbrains.annotations.Nullable;
@@ -4335,7 +4335,7 @@ index cf1733bc76d1e29ad0f533f6e49818f83e8e3358..f2a6f62f0344684668febc0999b81748
      private Material type = Material.AIR;
      private int amount = 0;
      private MaterialData data = null;
-@@ -614,4 +614,21 @@ public class ItemStack implements Cloneable, ConfigurationSerializable, Translat
+@@ -617,4 +617,21 @@ public class ItemStack implements Cloneable, ConfigurationSerializable, Translat
      public String getTranslationKey() {
          return Bukkit.getUnsafe().getTranslationKey(this);
      }