Update dependency NServiceBus.Callbacks to 6.0.0 (#817)

Co-authored-by: dependencyupdates[bot] <218638057+dependencyupdates[bot]@users.noreply.github.com>

Author: dependencyupdates[bot]

src/NServiceBus.Gateway.AcceptanceTests/NServiceBus.Gateway.AcceptanceTests.csproj

@@ -18,7 +18,7 @@
 
   <ItemGroup>
     <PackageReference Include="NServiceBus.AcceptanceTesting" Version="10.0.0" />
-    <PackageReference Include="NServiceBus.Callbacks" Version="6.0.0-alpha.5" />
+    <PackageReference Include="NServiceBus.Callbacks" Version="6.0.0" />
   </ItemGroup>
 
   <ItemGroup>