Bump System.Collections.Immutable from 8.0.0 to 9.0.0 (#465)

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

Directory.Packages.props

@@ -42,7 +42,7 @@
     <PackageVersion Include="NUnit" Version="4.2.2" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
     <PackageVersion Include="vswhere" Version="3.1.7" />
-    <PackageVersion Include="System.Collections.Immutable" Version="8.0.0" />
+    <PackageVersion Include="System.Collections.Immutable" Version="9.0.0" />
     <PackageVersion Include="System.Configuration.ConfigurationManager" Version="8.0.1" />
     <PackageVersion Include="System.Formats.Asn1" Version="8.0.1" />
     <PackageVersion Include="System.Reflection.Metadata" Version="8.0.1" />