Qt Android 6 with CMake is relying on deprecated ndk-bundle

The CMake build for Android added a feature to detect ANDROID_NDK_ROOT (which is used to detect the Android toolchain file) from ANDROID_SDK_ROOT. It tries to look for the ndk-bundle folder which has the default ndk package. However, the ndk-bundle is currently marked as deprecated (obsolete) which means it might be removed in the (near) future. 

It should be changed to detect the ndk from the <sdk_root>/ndk/<versions> instead.

Different Qt for Android versions depend on different ndk versions, for Qt 6, ndk r20 and r21 can be used (each has a few sub-versions). Ideally, we could have a CMake function that handles this auto-detection and account for the different ndk versions, and use the latest version found that is supported for a given Qt version.

The patch that introduced the feature: https://codereview.qt-project.org/c/qt/qtbase/+/317248.