Add vagrant machine for Ubuntu 20.04

Add vagrant machine for Ubuntu 20.04

The instructions in
[`VAGRANT.md`](https://github.com/osm-search/Nominatim/blob/42c80893cb1038e21c05c2d0d30e3a7a20f0fa26/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.