Update System.Reactive requirement from 5.* to 6.* (graphql-dotnet#3627)

Updates the requirements on [System.Reactive](https://github.com/dotnet/reactive) to permit the latest version.
- [Release notes](https://github.com/dotnet/reactive/releases)
- [Commits](https://github.com/dotnet/reactive/commits)

---
updated-dependencies:
- dependency-name: System.Reactive
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/GraphQL.DataLoader.Tests/GraphQL.DataLoader.Tests.csproj

@@ -20,7 +20,7 @@
   <ItemGroup>
     <PackageReference Include="Bogus" Version="34.*" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="3.1.*" />
-    <PackageReference Include="System.Reactive" Version="5.*" />
+    <PackageReference Include="System.Reactive" Version="6.*" />
   </ItemGroup>
 
   <ItemGroup>

src/GraphQL.MicrosoftDI.Tests/GraphQL.MicrosoftDI.Tests.csproj

@@ -16,7 +16,7 @@
   <ItemGroup>
     <PackageReference Include="Bogus" Version="34.*" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="3.1.*" />
-    <PackageReference Include="System.Reactive" Version="5.*" />
+    <PackageReference Include="System.Reactive" Version="6.*" />
   </ItemGroup>
 
   <ItemGroup>

src/GraphQL.Tests/GraphQL.Tests.csproj

@@ -35,7 +35,7 @@
     <PackageReference Include="Microsoft.Extensions.Hosting" Version="7.0.*" />
     <PackageReference Include="OpenTelemetry.Exporter.InMemory" Version="1.4.0" />
     <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.4.0" />
-    <PackageReference Include="System.Reactive" Version="5.*" />
+    <PackageReference Include="System.Reactive" Version="6.*" />
   </ItemGroup>
 
 </Project>