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>
diff --git a/MAINTAINERS b/MAINTAINERS
index c863e41..062ff39 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -534,7 +534,7 @@
 Releases
 ========
 
-2.3                                     Michael Niedermayer
+2.4                                     Michael Niedermayer
 2.2                                     Michael Niedermayer
 1.2                                     Michael Niedermayer