[RFC] Updating the CI image and bumping requirements

[Jonas Hahnfeld]

Hi all,

with 2.23.0 out, I'd like to update the CI image. The current one is
based on Ubuntu 16.04 which will be EOL in April, a bit more than 2
months from now. I think there are two logical replacements:
 * Ubuntu 18.04, maintained until 2023-04
 * Debian 9 Stretch, maintained until 2022-06

Neither of them has Guile 1.8 packaged, so that'll have to be compiled
when building the image anyway, but I'd like to go with Ubuntu 18.04
because it has Guile 2.2. In contrast, Debian 9 Stretch only comes with
guile-2.0 and I was surprised to find that LilyPond master doesn't even
compile with 2.0 at the moment [1].
However, there's another consideration: Ubuntu 18.04 comes with Python
3.6 and support for Python 3.5 (the current minimum & in Ubuntu 16.04)
is likely to break without proper testing. For that reason, I think we
should bump the requirement accordingly. The downside is that doing so
would effectively drop support for Debian 9 Stretch with Python 3.5. I
think that's ok given that Debian 10 Buster is the current version, but
wanted to check if that's a problem for anybody?

[If we agree on Ubuntu 18.04 as a kind of "minimum distribution", it
might make sense to bump some other requirements (such as Pango to get
rid of some compatibility code), but this can be done in due time.]

Cheers
Jonas


1: Overlay_string_port relies on scm_c_make_port_with_encoding which
doesn't exist in Guile 2.0. I haven't looked if there's a replacement,
but I find it unlikely and think the time would be better spent on a
working version with Guile 2.2.

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