Bump xunit.runner.visualstudio and System.Collections.Immutable

Bumps xunit.runner.visualstudio and System.Collections.Immutable. These dependencies needed to be updated together.

Updates `xunit.runner.visualstudio` from 2.5.3 to 2.8.1

Updates `System.Collections.Immutable` from 1.2.0 to 1.5.0

---
updated-dependencies:
- dependency-name: xunit.runner.visualstudio
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: System.Collections.Immutable
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Directory.Packages.props

@@ -12,7 +12,7 @@
         <PackageVersion Include="System.Text.Encodings.Web" Version="4.5.1" />
         <PackageVersion Include="System.Threading.Tasks.Extensions" Version="4.4.0" />
         <PackageVersion Include="xunit" Version="2.8.1" />
-        <PackageVersion Include="xunit.runner.visualstudio" Version="2.5.3" />
+        <PackageVersion Include="xunit.runner.visualstudio" Version="2.8.1" />
     </ItemGroup>
 
     <ItemGroup>