Bump ApprovalTests from 4.3.0 to 4.4.0 in /Rx.NET/Source

Bumps [ApprovalTests](https://github.com/approvals/ApprovalTests.Net) from 4.3.0 to 4.4.0.
- [Release notes](https://github.com/approvals/ApprovalTests.Net/releases)
- [Commits](approvals/ApprovalTests.Net@4.3.0...4.4.0)

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

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

@@ -31,7 +31,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.3.0" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
-    <PackageReference Include="ApprovalTests" Version="4.3.0" />
+    <PackageReference Include="ApprovalTests" Version="4.4.0" />
     <PackageReference Include="DiffPlex" Version="1.4.4" />
     <PackageReference Include="PublicApiGenerator" Version="9.3.0" />
   </ItemGroup>