Bump ApprovalTests from 3.0.14 to 4.0.0 in /Rx.NET/Source

Bumps [ApprovalTests](https://github.com/approvals/ApprovalTests.Net) from 3.0.14 to 4.0.0.
- [Release notes](https://github.com/approvals/ApprovalTests.Net/releases)
- [Commits](https://github.com/approvals/ApprovalTests.Net/commits)

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

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

@@ -38,7 +38,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
-    <PackageReference Include="ApprovalTests" Version="3.0.14" />
+    <PackageReference Include="ApprovalTests" Version="4.0.0" />
     <PackageReference Include="DiffPlex" Version="1.4.4" />
     <PackageReference Include="PublicApiGenerator" Version="7.0.1" />
   </ItemGroup>