Default to NDK 21.4.7075529

Context: https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops&tabs=yaml

Trying to use NDK 23.2.8568313 results in numerous xamarin-android
unit test failures, and thus doesn't work.  (I think `readelf` is
missing as well.)

Set NDK to 21.4.7075529, chosen because it's the default NDK version
on the Azure pipeline.

Author: jonpryor

src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.Versions.props

@@ -14,7 +14,7 @@
     <AndroidSdkPlatformToolsVersion Condition="'$(AndroidSdkPlatformToolsVersion)' == ''">33.0.2</AndroidSdkPlatformToolsVersion>
     <AndroidSdkEmulatorVersion Condition="'$(AndroidSdkEmulatorVersion)' == ''"></AndroidSdkEmulatorVersion>
     <AndroidSdkPlatformVersion Condition="'$(AndroidSdkPlatformVersion)' == ''">android-32</AndroidSdkPlatformVersion>
-    <AndroidNdkVersion Condition="'$(AndroidNdkVersion)' == ''">23.2.8568313</AndroidNdkVersion>
+    <AndroidNdkVersion Condition="'$(AndroidNdkVersion)' == ''">21.4.7075529</AndroidNdkVersion>
 
     <!-- obsolete; should consider removing eventually -->
     <AndroidSdkToolsVersion Condition="'$(AndroidSdkToolsVersion)' == ''">26.1.1</AndroidSdkToolsVersion>