chore(deps): update System.Reactive requirement from 5.* to 6.* in /src (#264)

Updates the requirements on [System.Reactive](https://github.com/dotnet/reactive) to permit the latest version.
- [Release notes](https://github.com/dotnet/reactive/releases)
- [Commits](https://github.com/dotnet/reactive/commits)

---
updated-dependencies:
- dependency-name: System.Reactive
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/Punchclock/Punchclock.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <PackageId>Punchclock</PackageId>
@@ -8,7 +8,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Reactive" Version="5.*" />
+    <PackageReference Include="System.Reactive" Version="6.*" />
   </ItemGroup>
 
 </Project>