build(deps): bump System.Collections.Immutable in /dotnet

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] <[email protected]>

dotnet/src/StackExchange.StacksIcons.csproj

@@ -41,6 +41,6 @@
 
 	<ItemGroup>
 		<FrameworkReference Include="Microsoft.AspNetCore.App" />
-		<PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
+		<PackageReference Include="System.Collections.Immutable" Version="9.0.0" />
 	</ItemGroup>
 </Project>