Bump Microsoft.Bcl.AsyncInterfaces from 5.0.0 to 6.0.0 in /Ix.NET/Source

Bumps [Microsoft.Bcl.AsyncInterfaces](https://github.com/dotnet/runtime) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](https://github.com/dotnet/runtime/compare/v5.0.0...v6.0.0)

---
updated-dependencies:
- dependency-name: Microsoft.Bcl.AsyncInterfaces
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Ix.NET/Source/System.Linq.Async/System.Linq.Async.csproj

@@ -26,7 +26,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.1' and '$(TargetFramework)' != 'netstandard2.1' " Include="Microsoft.Bcl.AsyncInterfaces" Version="5.0.0" />
+    <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.1' and '$(TargetFramework)' != 'netstandard2.1' " Include="Microsoft.Bcl.AsyncInterfaces" Version="6.0.0" />
     <ReferenceAssemblyProjectReference Include="..\refs\System.Linq.Async.Ref\System.Linq.Async.Ref.csproj" />
     <ProjectReference Include="..\System.Linq.Async.SourceGenerator\System.Linq.Async.SourceGenerator.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false" />
   </ItemGroup>

Ix.NET/Source/refs/System.Linq.Async.Ref/System.Linq.Async.Ref.csproj

@@ -10,7 +10,7 @@
 
   <ItemGroup>
     <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.1' and '$(TargetFramework)' != 'netstandard2.1' " 
-                      Include="Microsoft.Bcl.AsyncInterfaces" Version="5.0.0" />
+                      Include="Microsoft.Bcl.AsyncInterfaces" Version="6.0.0" />
     <ProjectReference Include="..\..\System.Linq.Async.SourceGenerator\System.Linq.Async.SourceGenerator.csproj"
                       OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false">
       <!-- Remove all the properties set by ReferenceAssemblyProjectReference, because they cause build failures. -->