Adjust our sources.list now that Jessie is stable

Originally created by @sajolida on #9281 (Redmine)

In config/chroot_sources/testing.chroot we have:

deb http://ftp.us.debian.org/debian/ jessie main contrib non-free

This should be changed to Jessie now that it’s stable.

Marking this as elevated as this can prevent additional software packages, and is thus a regression.

Edited May 21, 2020 by sajolida