Upgrade Halibut to 8.1.1503 (#1134)

Author: LukeButters

source/Octopus.Tentacle.Contracts/Octopus.Tentacle.Contracts.csproj

@@ -28,7 +28,7 @@
     </Otherwise>
   </Choose>
   <ItemGroup>
-    <PackageReference Include="Halibut" Version="8.1.1485" />
+    <PackageReference Include="Halibut" Version="8.1.1503" />
   </ItemGroup>
   <ItemGroup>
     <Compile Include="..\Solution Items\SolutionInfo.cs">