CI: disable EOL Bionic workflow

[scpeters]

🦟 Bug fix

Fixes failing CI workflows

Summary

Ubuntu Bionic (18.04) is now end-of-life, so disable that workflow, and run codecov and doxygen testing on Focal (20.04) instead.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• 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 and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #547 (4d8d9b6) into ign-math6 (d0b6179) will increase coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 4d8d9b6 differs from pull request most recent head 2cad637. Consider uploading reports for the commit 2cad637 to get more accurate results

@@              Coverage Diff              @@
##           ign-math6     #547      +/-   ##
=============================================
+ Coverage      99.68%   99.72%   +0.04%     
=============================================
  Files             73       74       +1     
  Lines           6918     6921       +3     
=============================================
+ Hits            6896     6902       +6     
+ Misses            22       19       -3     

see 3 files with indirect coverage changes