doc/APIchanges: Add 6.0 cut point

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

doc/APIchanges

@@ -2,6 +2,8 @@ The last version increases of all libraries were on 2023-02-09
 
 API changes, most recent first:
 
+-------- 8< --------- FFmpeg 6.0 was cut here -------- 8< ---------
+
 2023-02-16 - 927042b409 - lavf 60.2.100 - avformat.h
   Deprecate AVFormatContext io_close callback.
   The superior io_close2 callback should be used instead.