Bump xunit from 2.6.3 to 2.6.4

Bumps [xunit](https://github.com/xunit/xunit) from 2.6.3 to 2.6.4.
- [Commits](xunit/xunit@2.6.3...2.6.4)

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

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

ProtobufSourceGenerator.Tests/ProtobufSourceGenerator.Tests.csproj

@@ -12,7 +12,7 @@
     <PackageReference Include="protobuf-net" Version="3.2.30" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp.SourceGenerators.Testing.XUnit" Version="1.1.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
-    <PackageReference Include="xunit" Version="2.6.3" />
+    <PackageReference Include="xunit" Version="2.6.4" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.5" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" PrivateAssets="all" />
     <PackageReference Include="coverlet.collector" Version="6.0.0" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" PrivateAssets="all" />
     <PackageReference Include="Microsoft.CodeAnalysis.Common" Version="4.8.0" PrivateAssets="all" />