Bump System.Reactive.Compatibility from 5.0.0 to 6.0.0

Bumps [System.Reactive.Compatibility](https://github.com/dotnet/reactive) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/dotnet/reactive/releases)
- [Commits](https://github.com/dotnet/reactive/commits)

---
updated-dependencies:
- dependency-name: System.Reactive.Compatibility
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

tests/IntegrationTestServer/IntegrationTestServer.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk.Web">
+<Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
     <TargetFramework>net8</TargetFramework>
@@ -13,7 +13,7 @@
   <ItemGroup>
     <PackageReference Include="GraphQL.Server.All" Version="7.4.0" />
     <PackageReference Include="System.Reactive" Version="5.0.0" />
-    <PackageReference Include="System.Reactive.Compatibility" Version="5.0.0" />
+    <PackageReference Include="System.Reactive.Compatibility" Version="6.0.0" />
   </ItemGroup>
 
   <ItemGroup>