This avoids failing if building in an environment where texinfo
exists, but is a newer version. (Support for texinfo 7.1 and 7.2
was added in ffmpeg 8.0.)
Ubuntu 26.04 contains a version of texinfo that fails to build
the docs in ffmpeg 6.0.2. The "ubuntu-latest" runners on Github
Actions are now running on Ubuntu 26.04.
Martin Storsjö
09c763a2cb