LP1940146: Debian Bullseye Installation Support

LP1940146: Debian Bullseye Installation Support

This commit also addresses LP 1940145: Remove Installation
Support for Debian Jessie. The individual distribution-specific
targets (stretch, buster, etc.) have also been removed since
they are now all the same, much like both Ubuntu targets just
call ubuntu_generic. The two generic targets have been left
in case they're necessary again in future.

Signed-off-by: Jason Boyer <JBoyer@equinoxOLI.org>
Signed-off-by: Jason Stephenson <jason@sigio.com>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>