[master] Workflow updates

[chapulina]

• Add labeler workflow
• Add triage workflow
• Add Ubuntu CI workflow with Codecov upload
• Update badges
• Add CODEOWNERS
• This needs Script hooks gazebo-tooling/action-gz-ci#4 for the examples compilation to work.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@452d4a3). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #64   +/-   ##
=========================================
  Coverage          ?   73.81%           
=========================================
  Files             ?       67           
  Lines             ?     9204           
  Branches          ?        0           
=========================================
  Hits              ?     6794           
  Misses            ?     2410           
  Partials          ?        0           

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[maryaB-osr]

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