👩‍🌾 Remove bitbucket-pipelines

[chapulina]

🦟 Bug fix

Fixes gazebo-tooling/release-tools#203

Summary

This will make CI default to Bionic and shouldn't change anything about CI for all release branches, which are already using Bionic.

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

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https://github.com/osrf/buildfarmer/issues/224

[codecov-commenter]

Codecov Report

Merging #674 (0992032) into sdf6 (f13264e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             sdf6     #674   +/-   ##
=======================================
  Coverage   65.70%   65.70%           
=======================================
  Files          88       88           
  Lines       10163    10163           
=======================================
  Hits         6678     6678           
  Misses       3485     3485           

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[chapulina]

Also used the opportunity to backport the labeler and triage, which I noticed weren't working when I opened this PR: 0992032

[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