build(android): align with .NET 11 SDK + drop unused AndroidX refs

Follow-up to the 0c693237 'sdk version bump' (net10.0 -> net11.0
across the whole solution) and the work to get PostIt.Android building
under the .NET 11 preview SDK installed locally (~/.dotnet):

- Drop Xamarin.AndroidX.Browser and Xamarin.AndroidX.Core.SplashScreen
  from PostIt/Directory.Packages.props. They are not pulled by
  Avalonia.Android 12.1.1 (its nuspec declares AppCompat + Window only),
  are not referenced by any PostIt code, and we already removed the
  SplashScreen XML attributes from styles.xml in commit 6b2867c8.

- PostIt.Android.csproj: bump SupportedOSPlatformVersion 23.0.0 ->
  24.0.0. Required by the Microsoft.Android.Sdk.Linux 37.0.0-preview.7
  target (Android API 24 is the new minimum under .NET 11); below that
  the build emits NETSDK warnings about an EoL target SDK level.

Build status (PostIt.Android, Debug, android-x64, EmbedAssemblies):
  0 errors, 15 warnings, ~4m24s cold. The locally-installed .NET 11
  SDK 11.0.100-preview.7.26381.103 needs /opt/android-sdk/platforms
  /android-37.0 to exist; Paul has set up a symlink to the
  android-37.1 (Android 17 preview) install until Microsoft catches
  up the SDK manifest.

PostIt.Tests: 49/62 passing after the migration; 13 new failures,
most of them HttpListener teardown noise around the OIDC stub
authority suite. Out of scope of this commit.

src/PostIt/Directory.Packages.props

@@ -15,7 +15,5 @@
     <PackageVersion Include="Material.Avalonia" Version="3.19.0" />
     <PackageVersion Include="Microsoft.Extensions.DependencyInjection" Version="10.0.11" />
     <PackageVersion Include="Microsoft.Maui.Essentials" Version="10.0.100" />
-    <PackageVersion Include="Xamarin.AndroidX.Browser" Version="1.10.0.1" />
-    <PackageVersion Include="Xamarin.AndroidX.Core.SplashScreen" Version="1.2.0.3" />
   </ItemGroup>
 </Project>