chore(deps): update xunit-dotnet monorepo

| datasource | package                   | from  | to    |
| ---------- | ------------------------- | ----- | ----- |
| nuget      | xunit                     | 2.6.2 | 2.6.3 |
| nuget      | xunit.runner.visualstudio | 2.5.4 | 2.5.5 |

Author: renovate[bot]

UnitTests/UnitTests.csproj

@@ -10,8 +10,8 @@
 
     <ItemGroup>
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
-        <PackageReference Include="xunit" Version="2.6.2" />
-        <PackageReference Include="xunit.runner.visualstudio" Version="2.5.4" />
+        <PackageReference Include="xunit" Version="2.6.3" />
+        <PackageReference Include="xunit.runner.visualstudio" Version="2.5.5" />
         <ProjectReference Include="..\CSSParser\CSSParser.csproj" />
         <ProjectReference Include="..\CSSParser.ExtendedLESSParser\CSSParser.ExtendedLESSParser.csproj" />
     </ItemGroup>