build(deps): update ReactiveUI.AndroidX requirement in /src (#333)

Updates the requirements on [ReactiveUI.AndroidX](https://github.com/reactiveui/reactiveui) to permit the latest version.
- [Release notes](https://github.com/reactiveui/reactiveui/releases)
- [Commits](reactiveui/ReactiveUI@14.1.1...15.1.1)

---
updated-dependencies:
- dependency-name: ReactiveUI.AndroidX
  dependency-type: direct:production
...

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

src/ReactiveUI.Validation.AndroidX/ReactiveUI.Validation.AndroidX.csproj

@@ -11,7 +11,7 @@
 
   <ItemGroup>
     <PackageReference Include="Xamarin.Google.Android.Material" Version="1.*" />
-    <PackageReference Include="ReactiveUI.AndroidX" Version="14.*" />
+    <PackageReference Include="ReactiveUI.AndroidX" Version="15.*" />
   </ItemGroup>
 
   <ItemGroup>