Allow MaximumCompatibleNDKMajorVersion=28

[jonathanpeppers]

Context: dotnet/android@80559b5

We are already using the 28.x Android NDK, so we should allow it here.

I also updated a unit test that was outdated.

[Copilot]

Pull Request Overview

This PR updates the maximum compatible Android NDK major version from 26 to 28, aligning the code with the actual NDK version already in use. The change ensures that the system can properly recognize and work with NDK 28.x versions.

Key changes:

• Updated MaximumCompatibleNDKMajorVersion constant from 26 to 28 in production code
• Enhanced unit test to include NDK versions 26-28 and verify correct version selection logic

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkBase.cs	Updated the maximum compatible NDK version constant from 26 to 28
tests/Xamarin.Android.Tools.AndroidSdk-Tests/AndroidSdkInfoTests.cs	Updated test constant and added NDK versions 26-28 to test array; changed expected version logic to use second-to-last element

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