It has been deprecated in b4f59bee,
but the attribute_deprecated was not set and there was no entry
in APIchanges. This commit adds these and schedules it for removal.
Given that the reason behind the deprecation is exactly the same
as in av_fopen_utf8(), reuse its FF_API_AV_FOPEN_UTF8.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>