Bump xunit from 2.7.1 to 2.8.1

Bumps [xunit](https://github.com/xunit/xunit) from 2.7.1 to 2.8.1.
- [Commits](xunit/xunit@2.7.1...2.8.1)

---
updated-dependencies:
- dependency-name: xunit
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

tests/Maths/Silk.NET.Maths.Tests.csproj

@@ -12,7 +12,7 @@
 
     <ItemGroup>
       <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
-      <PackageReference Include="xunit" Version="2.7.1" />
+      <PackageReference Include="xunit" Version="2.8.1" />
       <PackageReference Include="xunit.runner.visualstudio" Version="2.8.1">
         <PrivateAssets>all</PrivateAssets>
         <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>