build(deps): update ReactiveUI requirement from 17.* to 18.* in /samples (#469)

Updates the requirements on [ReactiveUI](https://github.com/reactiveui/reactiveui) to permit the latest version.
- [Release notes](https://github.com/reactiveui/reactiveui/releases)
- [Commits](reactiveui/ReactiveUI@17.0.1...18.2.5)

---
updated-dependencies:
- dependency-name: ReactiveUI
  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]

samples/LoginApp.Uwp/LoginApp.Uwp.csproj

@@ -149,7 +149,7 @@
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.NETCore.UniversalWindowsPlatform" Version="6.2.14" />
-    <PackageReference Include="ReactiveUI" Version="17.*" />
+    <PackageReference Include="ReactiveUI" Version="18.*" />
     <PackageReference Include="ReactiveUI.Uwp">
       <Version>18.2.5</Version>
     </PackageReference>

samples/LoginApp/LoginApp.csproj

@@ -4,7 +4,7 @@
     <LangVersion>latest</LangVersion>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="ReactiveUI" Version="17.*" />
+    <PackageReference Include="ReactiveUI" Version="18.*" />
     <PackageReference Include="ReactiveUI.Fody" Version="18.*" />
     <PackageReference Include="ReactiveUI.Validation" Version="2.*" />
   </ItemGroup>