Move Github Actions VMs to Ubuntu 18

[kevinclark19a]

Description

python2.6 previously required ubuntu 16 as later ubuntu versions did not support 2.6 in GHA. We have since moved to installing python2.6 from a frozen tar ball, and later ubuntus can be used. Unfortunately, python2.6 fails with a missing crypto module (_md5) on ubuntu-latest (currently ubuntu 20), and python3.4 fails on ubuntu-latest because it is not provided by actions/setup-python@v2.

ubuntu 16 is reaching EOL for GHA and must be replaced come September 2021.

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PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made

Quality of Code and Contribution Guidelines

• I have read the contribution guidelines.

[codecov]

Codecov Report

Merging #2291 (83112b8) into develop (94890a0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #2291   +/-   ##
========================================
  Coverage    70.60%   70.60%           
========================================
  Files           96       96           
  Lines        13903    13903           
  Branches      1986     1986           
========================================
  Hits          9816     9816           
  Misses        3654     3654           
  Partials       433      433           

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