Changelog: Add 4.2 cut marker

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 4.2:
 - tpad filter
 - AV1 decoding support through libdav1d
 - dedot filter