Changelog/RELEASE_NOTES: prepare for 2.4
MAINTAINERS: Add 2.4 to maintained releases, drop 2.3

2.2 is used by 4 distributions, 2.3 by none, thus continuing maintaining
2.2 makes more sense than 2.3 see:
https://trac.ffmpeg.org/wiki/Downstreams

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>