CI: Update to Ubuntu 18.04
CI: Update to Ubuntu 18.04

Follow OpenTitan's lead and move to Ubuntu 18.04 for the CI runner.
Ubuntu 16.04 comes with Python 3.5, which is EOL and causes more and
more issues.

Specificially, this update is triggered by rst2pdf not being installable
with pip's new dependency resolver (see also
https://github.com/rst2pdf/rst2pdf/issues/489#issuecomment-736652808).

AmbiML/ibex,lowRISC/ibex,lowRISC/ibex,lowRISC/ibex,AmbiML/ibex,AmbiML/ibex,AmbiML/ibex,lowRISC/ibex