Bump PublicApiGenerator from 7.0.1 to 9.1.0 in /Rx.NET/Source

Bumps [PublicApiGenerator](https://github.com/JakeGinnivan/ApiApprover) from 7.0.1 to 9.1.0.
- [Release notes](https://github.com/JakeGinnivan/ApiApprover/releases)
- [Commits](PublicApiGenerator/PublicApiGenerator@7.0.1...9.1.0)

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

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

@@ -40,7 +40,7 @@
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
     <PackageReference Include="ApprovalTests" Version="4.0.0" />
     <PackageReference Include="DiffPlex" Version="1.4.4" />
-    <PackageReference Include="PublicApiGenerator" Version="7.0.1" />
+    <PackageReference Include="PublicApiGenerator" Version="9.1.0" />
   </ItemGroup>
 
 </Project>