The instructions in
[`VAGRANT.md`](42c80893cb/VAGRANT.md)
still work as before. The names of the Vagrant machines are updated so
that Ubuntu 18.04 (previously called `ubuntu`) is now called `ubuntu18`
and Ubuntu 20.04 is now called `ubuntu20`.
The version changes from Ubuntu 18.04 to Ubuntu 20.04 are:
- Python: 3.6 to 3.8
- Postgres: 10 to 12
- PHP: 7.2 to 7.4
In the `apt-get`, I changed `--force` to `--allow-downgrades --allow-remove-essential --allow-change-held-packages`, because the former is deprecated. Cf. the [manpage of apt-get](http://manpages.ubuntu.com/manpages/focal/man8/apt-get.8.html)
The php module `codesniffer` was previously installed via Composer, but it is available via the Ubuntu repository, so I installed it via `apt-get` now.
Basically leaves the part that needs to be done by the automatic
vagrant script in the bash script and moves the remaining part
into an OS-independent md file.
Also fixes some of the instructions for CentOS. Now almost runs
through (some minor issue with permission remains).
