build(deps): bump Avalonia from 0.10.12 to 0.10.15 in /samples (#467)

Bumps [Avalonia](https://github.com/AvaloniaUI/Avalonia) from 0.10.12 to 0.10.15.
- [Release notes](https://github.com/AvaloniaUI/Avalonia/releases)
- [Commits](AvaloniaUI/Avalonia@0.10.12...0.10.15)

---
updated-dependencies:
- dependency-name: Avalonia
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

samples/LoginApp.Avalonia/LoginApp.Avalonia.csproj

@@ -5,7 +5,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Avalonia" Version="0.10.12" />
+        <PackageReference Include="Avalonia" Version="0.10.15" />
         <PackageReference Include="Avalonia.Desktop" Version="0.10.12" />
         <PackageReference Include="Avalonia.ReactiveUI" Version="0.10.15" />
         <PackageReference Include="MessageBox.Avalonia" Version="1.7.1" />