Add Installation Support for Debian 11.0 (Bullseye)

To answer Josh's question in bug 1940149 about installing Node.js from source, I'm inclined to continue doing so because we may want to change the version of Node.js that we use in the coming years.

Also, since a duplicate bug was opened, I'm setting this bug to confirmed.

Here's an OpenSRF branch that addresses both this and bug 1940145 (remove jessie support) because it simplifies the remaining make targets significantly. Basically every Debian distribution does the same thing now behind the scenes. I left the separate generic_debian and generic_ubuntu targets just in case they're useful in the near future but they're currently identical.

https://git.evergreen-ils.org/?p=working/OpenSRF.git;a=shortlog;h=refs/heads / working/user/jboyer/lp1940146_bullseye_support

I forgot to mention, there are 2 commits in that branch to Do The Thing, and an optional 3rd that removes the math xul client example since there isn't much point in that sort of thing anymore.

I'll review Jason Boyer's changes for OpenSRF and see about making the changes for Evergreen.

I have signed off on Jason Boyer's OpenSRF branch at https://git.evergreen-ils.org/?p=working/OpenSRF.git;a=shortlog;h=refs/heads/user/dyrcona/lp1940146_bullseye_support-signoff

NOTE that I have also added 1 commit on top to modify the README in order to clarify where a couple of ejabberd changes are required.

I have also pushed a branch for Evergreen to https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/dyrcona/lp1940146_bullseye_support

This branch has 3 commits:

* The first is for Jason Boyer's fix for bug 1850156
* The second removes Debian Jessie per bug 1940145
* The third contains all of the changes for this bug.

I have tested this branch on a VM running Debian Bullseye and all of the tests for Perl and PgTap pass.

I have also tested it on Ubuntu Bionic and Focal so far. Testing on Buster and Stretch remain for others, but I anticipate no problems there.

Noting bug 1944765 as a new loose end to include for Bullseye.

Pushed to OpenSRF and Evergreen master branches. Thanks, Jasons!