Use Ubuntu bionic in CI

[scpeters]

🦟 Bug fix

Fixes Ubuntu Jenkins builds on sdf6 branch

Summary

Our Jenkins Ubuntu builds use the docker image name specified in the bitbucket-pipelines.yml file (see sdformat.dsl, but Ubuntu xenial is now EOL, so update it to bionic. See Ubuntu test failures in #500

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[codecov-commenter]

Codecov Report

Merging #626 (4416774) into sdf6 (6deeb06) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             sdf6     #626   +/-   ##
=======================================
  Coverage   65.70%   65.70%           
=======================================
  Files          88       88           
  Lines       10163    10163           
=======================================
  Hits         6678     6678           
  Misses       3485     3485           

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-25-fortress-edifice-citadel/1343/1