minimum Meson version

meson.build currently says

# We want < 0.56 for python 3.5 compatibility on old platforms. EPEL for
# RHEL 7 has 0.55. < 0.54 would require replacing some uses of the fs
# module, < 0.53 all uses of fs. So far there's no need to go to >=0.56.
meson_version: '>=0.54',

Since the current minimum supported Python version is now actually 3.6,
we could update this a bit.

The first Meson version to require Python 3.7 is 0.62, so we should stay
below that.

Moving to 0.55 and 0.56 would get rid of some future-deprecated warnings.

There is some conditional code for 0.57 and 0.59, so landing on either
of these would allow getting rid of some of that.

I see that Rocky Linux 8 ships with Meson 0.58.2 [0], so maybe that is a
good target to aim for. (I don't know if that carried over from RHEL 8
or is their own doing.) But there aren't any compelling features new in
0.58 (format strings seem nice but are pretty much cosmetic), so maybe
setting the minimum to 0.57 is enough.

[0]: https://dl.rockylinux.org/pub/rocky/8/Devel/x86_64/os/Packages/m/