build(deps): bump Verify.Xunit from 19.12.0 to 20.4.0 (#181)

Bumps [Verify.Xunit](https://github.com/VerifyTests/Verify) from 19.12.0 to 20.4.0.
- [Release notes](https://github.com/VerifyTests/Verify/releases)
- [Commits](VerifyTests/Verify@19.12.0...20.4.0)

---
updated-dependencies:
- dependency-name: Verify.Xunit
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/Splat.DependencyInjection.SourceGenerator.Tests/Splat.DependencyInjection.SourceGenerator.Tests.csproj

@@ -14,7 +14,7 @@
     <PackageReference Include="coverlet.msbuild" Version="6.0.0" />
     <PackageReference Include="Verify.SourceGenerators" Version="1.5.0" />
     <PackageReference Include="ReactiveMarbles.SourceGenerator.TestNuGetHelper" Version="1.2.21" />
-    <PackageReference Include="Verify.Xunit" Version="19.12.0" />
+    <PackageReference Include="Verify.Xunit" Version="20.4.0" />
     <PackageRefernece Include="Microsoft.CodeAnalysis.CSharp" Version="3.10.0" PrivateAssets="all" />
   </ItemGroup>