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

MAINTAINERS

@@ -534,7 +534,7 @@ x86                                     Michael Niedermayer
 Releases
 ========
 
-2.3                                     Michael Niedermayer
+2.4                                     Michael Niedermayer
 2.2                                     Michael Niedermayer
 1.2                                     Michael Niedermayer