Debian Stretch repositories has been archived

As Debian Stretch package repositories have disappeared from debian mirrors, the default upgrade path for Wazo 19.12 is broken.
In order to support upgrade from 19.12 to newer buster-based releases, the wazo-dist-upgrade package has been updated.
That upgrade path also requires manually changing the default debian mirrors to alternative archive mirrors.
For example:

# /etc/apt/sources.list
deb stretch main
deb-src stretch main