Stop building packages for Debian 8 and Raspbian Stretch

Debian 8 and Raspbian Stretch are both EOL from June 2020. This effectively means that 13.1 was the last supported version for these two OS versions.

Tasks:

Remove the related jobs from pipeline (!4430 (merged))
Add upcoming deprecation warning in release post (gitlab-com/www-gitlab-com!57189 (merged))
Add deprecation notice in release post (gitlab-com/www-gitlab-com!58632 (merged))
Remove from install page (gitlab-com/www-gitlab-com!57178 (merged))

Edited Sep 10, 2020 by Larissa Lane