Bump Microsoft.Maui.Controls.Compatibility in /Source (#4381)

Bumps [Microsoft.Maui.Controls.Compatibility](https://github.com/dotnet/maui) from 9.0.12 to 9.0.21.
- [Release notes](https://github.com/dotnet/maui/releases)
- [Changelog](https://github.com/dotnet/maui/blob/main/docs/ReleaseSchedule.md)
- [Commits](dotnet/maui@9.0.12...9.0.21)

---
updated-dependencies:
- dependency-name: Microsoft.Maui.Controls.Compatibility
  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>

Source/Csla.Xaml.Maui/Csla.Xaml.Maui.csproj

@@ -24,12 +24,12 @@
 
   <ItemGroup Condition="$(TargetFramework.StartsWith(&quot;net9&quot;))">
     <PackageReference Include="Microsoft.Maui.Controls" Version="9.0.21" />
-    <PackageReference Include="Microsoft.Maui.Controls.Compatibility" Version="9.0.12" />
+    <PackageReference Include="Microsoft.Maui.Controls.Compatibility" Version="9.0.21" />
     <PackageReference Include="Microsoft.Extensions.Logging.Debug" Version="9.0.0" />
   </ItemGroup>
   <ItemGroup Condition="$(TargetFramework.StartsWith(&quot;net8&quot;))">
     <PackageReference Include="Microsoft.Maui.Controls" Version="9.0.21" />
-    <PackageReference Include="Microsoft.Maui.Controls.Compatibility" Version="9.0.12" />
+    <PackageReference Include="Microsoft.Maui.Controls.Compatibility" Version="9.0.21" />
     <PackageReference Include="Microsoft.Extensions.Logging.Debug" Version="9.0.0" />
   </ItemGroup>