r18b is from 2018, now, r24 is releasing.

Changelogs can be found at https://github.com/android/ndk/wiki/Unsupported-Downloads (also https://developer.android.com/ndk/downloads/revision_history)

• Add a warning/error in the code if the ndk is too old.
• r21e is LTS from Jan 2021 (and is easy to integrate in our code).
• r23b the latest version LTS (but requires more updates to be fully integrated).

Maybe do it in 2 steps if one takes too much time.

/!\ https://invent.kde.org/education/gcompris/-/merge_requests/90 Qt may not have a version compatible with ndk r23b.