feat: add MC 1.21.6 (protocol 771) version constants and enum values

- Add protocol 771 constant MC_1_21_6_Version in Protocol18.cs
- Register 1.21.6 in ProtocolHandler (version map, supported list)
- Extend upper-bound checks from MC_1_21_5 to MC_1_21_6
- Add 19 new ItemType entries (16 colored Harnesses, DriedGhast,
  HappyGhastSpawnEgg, MusicDiscTears)
- Add HappyGhast to EntityType enum
- Add DriedGhast to Material enum
- Add new packet types: Waypoint, ClearDialog, ShowDialog (clientbound),
  ChangeGameMode, CustomClickAction (serverbound),
  ClearDialog, ShowDialog (config clientbound),
  CustomClickAction (config serverbound)

Made-with: Cursor

MinecraftClient/Mapping/EntityType.cs

@@ -79,6 +79,7 @@ namespace MinecraftClient.Mapping
         GlowSquid,
         Goat,
         Guardian,
+        HappyGhast,
         Hoglin,
         HopperMinecart,
         Horse,

MinecraftClient/Mapping/Material.cs

@@ -364,6 +364,7 @@ namespace MinecraftClient.Mapping
         DragonEgg,
         DragonHead,
         DragonWallHead,
+        DriedGhast,
         DriedKelpBlock,
         DripstoneBlock,
         Dropper,