DRAFT RFC docs: update recommended NDK -> r23c

[brodybits]

(no longer LTS)

ref:

• https://github.com/android/ndk/wiki/Changelog-r23#r23c
• https://github.com/android/ndk/wiki/Unsupported-Downloads

This proposal gets the latest SDK possible without dropping the existing Android 4.1 support.

I would personally favor dropping the super-old Jelly Bean support and recommending NDK r25 LTS version instead.