Changelog: rename next to 2.6

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

Changelog

 Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
-version <next>:
+version 2.6:
 - nvenc encoder
 - 10bit spp filter
 - colorlevels filter