Bump ReactiveUI.Testing and ReactiveUI (#578)

Bumps [ReactiveUI.Testing](https://github.com/reactiveui/reactiveui) and [ReactiveUI](https://github.com/reactiveui/reactiveui). These dependencies needed to be updated together.

Updates `ReactiveUI.Testing` from 19.6.1 to 20.1.1
- [Release notes](https://github.com/reactiveui/reactiveui/releases)
- [Commits](reactiveui/ReactiveUI@19.6.1...20.1.1)

Updates `ReactiveUI` from 19.6.12 to 20.1.1
- [Release notes](https://github.com/reactiveui/reactiveui/releases)
- [Commits](reactiveui/ReactiveUI@19.6.12...20.1.1)

---
updated-dependencies:
- dependency-name: ReactiveUI.Testing
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: ReactiveUI
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

tests/ImageSort.UnitTests/ImageSort.UnitTests.csproj

@@ -15,8 +15,8 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="ReactiveUI" Version="19.6.12" />
-    <PackageReference Include="ReactiveUI.Testing" Version="19.6.1" />
+    <PackageReference Include="ReactiveUI" Version="20.1.1" />
+    <PackageReference Include="ReactiveUI.Testing" Version="20.1.1" />
     <PackageReference Include="xunit" Version="2.8.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.7">
       <PrivateAssets>all</PrivateAssets>