[RFC] Switch Docker images to Ubuntu 20.04 and bump requirements

[Jonas Hahnfeld]

Hi all,

Ubuntu 18.04 will reach its end-of-life later this year, so I'd like to
propose that we switch our Docker images for testing and the official
documentation builds to Ubuntu 20.04. Here is a merge request:
https://gitlab.com/lilypond/lilypond/-/merge_requests/1827

One caveat is that I want to finally use the pre-packaged fonts-urw-
base35 instead of downloading and extracting it ourselves.
Unfortunately, the *.otf files are installed in a different place, so
we have to temporarily fix up some paths in the test-baseline. Another
"pain point" is that the images are much bigger (997MB compared to
823MB with Ubuntu 18.04). I had a quick look and most if it seems to
come from TexLive. If somebody has ideas how to "install less" or
otherwise reduce the size, I would welcome contributions.

As a second step, I would like to bump a number of requirements to
versions available in Ubuntu 18.04. This simplifies some checks and
also acknowledges the fact that we are rarely testing with older
versions, so any breakage may go unnoticed:
 - Python 3.6 -> 3.8 (this is particularly easy to break); the
exception would be the scripts in release/binaries/ that need to run on
CentOS 7, which only has Python 3.6
 - Fontconfig 2.4.0 -> 2.13
 - FreeType 2.3.9 -> 2.10
 - Glib2 2.38 -> 2.64
 - Pango 1.40 -> 1.44
 - Cairo 1.8.8 -> 1.16
 - Texinfo 6.1 -> 6.7

What I would like to keep are the versions of FontForge, where we are
bound by CentOS 7, and Ghostscript, because it had a licensing change
to AGPL and I think it would be good the keep the ability to run with a
GPL-only version.

Let me know what you think.

Jonas

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