Updated Upstream (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

CraftBukkit Changes:
f81f4994 SPIGOT-6594: NPE at Bukkit.getServer().getGenerateStructures()
295ab080 #872: SculkSensor accesses underlying TileEntity instead of snapshot for vibration frequency.

work/CraftBukkit

@@ -1 +1 @@
-Subproject commit 887bc05190637cc5fe5783c0cd353e4eb880a7f5
+Subproject commit f81f49940f2078c375e639c41cd5a109c73dc203