Bump System.Collections.Immutable from 5.0.0 to 6.0.0 (#211)

Bumps [System.Collections.Immutable](https://github.com/dotnet/runtime) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](dotnet/runtime@v5.0.0...v6.0.0)

---
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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Owen Smith <owen@omsmith.ca>

Directory.Packages.props

@@ -15,7 +15,7 @@
 		<PackageVersion  Include="NUnit3TestAdapter" Version="4.2.0" />
 		<PackageVersion  Include="RichardSzalay.MockHttp" Version="6.0.0" />
 		<PackageVersion  Include="SimpleLogInterface" Version="3.0.1" />
-		<PackageVersion  Include="System.Collections.Immutable" Version="5.0.0" />
+		<PackageVersion  Include="System.Collections.Immutable" Version="6.0.0" />
 		<PackageVersion  Include="System.IdentityModel.Tokens.Jwt" Version="6.14.1" />
 		<PackageVersion  Include="System.Text.Json" Version="6.0.1" />
 	</ItemGroup>