Changelog: replace <next> by 4.0

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

Changelog

 Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
-version <next>:
+version 4.0:
 - Bitstream filters for editing metadata in H.264, HEVC and MPEG-2 streams
 - Dropped support for OpenJPEG versions 2.0 and below. Using OpenJPEG now
   requires 2.1 (or later) and pkg-config.