Bump System.Collections.Immutable from 6.0.0 to 8.0.0

Bumps [System.Collections.Immutable](https://github.com/dotnet/runtime) from 6.0.0 to 8.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](dotnet/runtime@v6.0.0...v8.0.0)

---
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>

Author: dependabot[bot]

src/X.Spectator/X.Spectator.csproj

@@ -22,7 +22,7 @@
 
     <ItemGroup>
       <PackageReference Include="JetBrains.Annotations" Version="2021.3.0" />
-      <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
+      <PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
     </ItemGroup>
 
 </Project>