chore(deps): update dependency reactiveui.winforms to v20 (#88)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

src/ReactiveMvvm.WinForms/ReactiveMvvm.WinForms.csproj

@@ -6,7 +6,7 @@
     <UseWindowsForms>true</UseWindowsForms>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="ReactiveUI.WinForms" Version="19.6.12" />
+    <PackageReference Include="ReactiveUI.WinForms" Version="20.1.1" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\ReactiveMvvm\ReactiveMvvm.csproj" />