Switch to NDK21

cf. https://android-developers.googleblog.com/2019/10/introducing-ndk-r21-our-first-long-term.html

First NDK LTS. This NDK should be used for the next VLC-Android 3.3.x release. And specially for our last VLC-android 3.x release before the 4.0, since this release will be likely an LTS as it will be the last one to support old android APIs.

changed milestone to %3.3

added libVLC label

added 2 commits

• d49bdbfb - ci: Bump images to use NDK 21
• eb9b367b - Merge branch 'ndk21' into 'ndk21'

Compare with previous version

enabled an automatic merge when the pipeline for d49bdbfb succeeds

merged