Resolves #10636 (http://trac.ffmpeg.org/ticket/10636)

Texinfo 7.0, released in November 2022, changed the names of various
functions. Compiling docs with Texinfo 7.0 results in warnings and
improperly formatted documentation. More old names appear to have
been removed in Texinfo 7.1, released October 2023, which causes docs
compilation to fail.

This PR addresses the issue by adding logic to switch between the old
and new function names depending on the Texinfo version. Texinfo 6.8
produces identical documentation before and after the patch. The change
to the CSS makes the documentation generated by Texinfo >= 7.0 appear
more similar to that generated by Texinfo <= 6.8.

CC
https://www.mail-archive.com/debian-bugs-dist@lists.debian.org/msg1938238.html
https://bugs.gentoo.org/916104

Signed-off-by: Frank Plowman <post@frankplowman.com>

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