Bump coverlet.collector from 1.2.0 to 1.2.1 in /Rx.NET/Source

Bumps [coverlet.collector](https://github.com/tonerdo/coverlet) from 1.2.0 to 1.2.1.
- [Release notes](https://github.com/tonerdo/coverlet/releases)
- [Commits](https://github.com/tonerdo/coverlet/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Rx.NET/Source/Directory.build.props

@@ -33,7 +33,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(IsTestProject)' == 'true'">
-    <PackageReference Include="coverlet.collector" Version="1.2.0" />
+    <PackageReference Include="coverlet.collector" Version="1.2.1" />
   </ItemGroup>
 
 </Project>