Bump xunit from 2.6.5 to 2.6.6

Bumps [xunit](https://github.com/xunit/xunit) from 2.6.5 to 2.6.6.
- [Commits](xunit/xunit@2.6.5...2.6.6)

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

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

firely-validator-api-tests.props

@@ -21,7 +21,7 @@
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
 		<PackageReference Include="MSTest.TestAdapter" Version="3.1.1" />
 		<PackageReference Include="MSTest.TestFramework" Version="3.1.1" />
-		<PackageReference Include="xunit" Version="2.6.5" />
+		<PackageReference Include="xunit" Version="2.6.6" />
 		<PackageReference Include="xunit.runner.visualstudio" Version="2.5.6">
 			<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
 			<PrivateAssets>all</PrivateAssets>