Bump Verify.Xunit from 19.14.1 to 20.4.0 in /Rx.NET/Source (#1969)

Bumps [Verify.Xunit](https://github.com/VerifyTests/Verify) from 19.14.1 to 20.4.0.
- [Release notes](https://github.com/VerifyTests/Verify/releases)
- [Commits](https://github.com/VerifyTests/Verify/compare/19.14.1...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>

Rx.NET/Source/tests/Tests.System.Reactive.ApiApprovals/Tests.System.Reactive.ApiApprovals.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFrameworks>net48</TargetFrameworks>
@@ -26,7 +26,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.5.0" />
-    <PackageReference Include="Verify.Xunit" Version="19.14.1" />
+    <PackageReference Include="Verify.Xunit" Version="20.4.0" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" />
     <PackageReference Include="DiffPlex" Version="1.7.1" />