build(deps): update ReactiveUI.AndroidX requirement in /src (#535)

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@18.0.4...19.2.1)

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

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

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

@@ -1,4 +1,4 @@
-<Project Sdk="MSBuild.Sdk.Extras">
+<Project Sdk="MSBuild.Sdk.Extras">
 
   <PropertyGroup>
     <TargetFrameworks>MonoAndroid12.0;MonoAndroid12.1;MonoAndroid13.0;net6.0-android;net7.0-android</TargetFrameworks>
@@ -10,7 +10,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="ReactiveUI.AndroidX" Version="18.*" />
+    <PackageReference Include="ReactiveUI.AndroidX" Version="19.*" />
   </ItemGroup>
 
   <ItemGroup>