Bump System.Collections.Immutable from 8.0.0 to 9.0.0 in /src

Bumps [System.Collections.Immutable](https://github.com/dotnet/runtime) from 8.0.0 to 9.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](dotnet/runtime@v8.0.0...v9.0.0)

---
updated-dependencies:
- dependency-name: System.Collections.Immutable
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

src/TestFixture/TestFixture.csproj

@@ -14,7 +14,7 @@
   </PropertyGroup>  
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'net462' ">
-    <PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="9.0.0" />
     <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="8.0.0" />
   </ItemGroup>