Bump System.Reactive from 4.4.1 to 5.0.0

Bumps [System.Reactive](https://github.com/dotnet/reactive) from 4.4.1 to 5.0.0.
- [Release notes](https://github.com/dotnet/reactive/releases)
- [Commits](dotnet/reactive@rxnet-v4.4.1...ixnet-v5.0.0)

---
updated-dependencies:
- dependency-name: System.Reactive
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Octokit.Reactive/Octokit.Reactive.csproj

@@ -41,7 +41,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Reactive" Version="4.4.1" />
+    <PackageReference Include="System.Reactive" Version="5.0.0" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All"/>
   </ItemGroup>