build(deps): bump System.Collections.Immutable in /dotnet (#264)

Bumps [System.Collections.Immutable](https://github.com/dotnet/runtime) from 7.0.0 to 8.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](dotnet/runtime@v7.0.0...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] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

dotnet/src/StackExchange.StacksIcons.csproj

@@ -41,7 +41,7 @@
 
  <ItemGroup>
  <PackageReference Include="Microsoft.AspNetCore.Html.Abstractions" Version="2.2.0" />
- <PackageReference Include="System.Collections.Immutable" Version="7.0.0" />
+ <PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
  <PackageReference Include="System.Memory" Version="4.5.5" />
  </ItemGroup>
 </Project>