Bug#1034315: ffmpeg: 5.1, 6.0 libraries not co-installable

[Peter Samuelson <peters@p12n.org>]

Package: libavcodec60
Version: 7:6.0-1
Severity: minor

libavcodec{59,60} cannot be co-installed because both depend on exact
versions of libswresample4.  (Also, libavfilter{8,9} have the same
issue.)

This will require an all-or-nothing library transition, and it means I
can't install ffmpeg 6.0 from experimental without rebuilding many
other things.

It was not an issue with the previous ABI update, ffmpeg 7:4.x -> 7:5.x,
because libswresample SONAME was updated as well.