Remove Installation Support for Ubuntu 16.04 Xenial Xerus

[Jason Stephenson]

Since we're about to add support for Ubuntu 20.04 Focal Fossa and Ubuntu 16.04 goes out of Ubuntu community support in April 2021, we should remove installation support for it from Evergreen and OpenSRF.

[Jason Stephenson (jstephenson)]

Branches are available:

https://git.evergreen-ils.org/?p=working/OpenSRF.git;a=shortlog;h=refs/heads/user/dyrcona/lp1901899-remove-ubuntu-xenial-xerus

and

https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/dyrcona/lp1901899-remove-ubuntu-xenial-xerus

The above branches are based on the branches from bug 1875544.

[Jason Stephenson (jstephenson)]

I have rebased the working branches on the rebased working branches from bug 1875544, and force pushed them to keep up with the latest feedback fest additions.

[Jason Stephenson (jstephenson)]

The Evergreen branch was again rebased on the branch from bug 1875544 to keep up with the latest changes.

[Ben Shum (bshum)]

Pushed to master for Evergreen / OpenSRF.