Bump System.Collections.Immutable from 1.4.0 to 1.6.0

Bumps [System.Collections.Immutable](https://github.com/dotnet/corefx) from 1.4.0 to 1.6.0.
- [Release notes](https://github.com/dotnet/corefx/releases)
- [Commits](https://github.com/dotnet/corefx/commits)

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

src/Hyperion.Tests/Hyperion.Tests.csproj

@@ -7,7 +7,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="$(TestSdkVersion)" />
-    <PackageReference Include="System.Collections.Immutable" Version="1.4.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="1.6.0" />
     <PackageReference Include="xunit" Version="$(XunitVersion)" />
     <PackageReference Include="xunit.runner.visualstudio" Version="$(XunitVersion)" />
     <DotNetCliToolReference Include="dotnet-xunit" Version="$(XunitVersion)" />