Update the PPA to support Ubuntu 20.04, drop 16.04

Summary:
The new Ubuntu LTS version 20.04 "Focal Fossa" is about to be released
(scheduled on 2020/04/23). This diff removes the old 16.04 version and
add the new 20.04 version to the debian-packages.sh script, so the PPA
will serve the next release for Focal.

Test Plan:

../contrib/release/debian-packages.sh --dry-run <your_email>

Check there is a focal source.changes file generated.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5802