CI workflow: use checkout v3

[scpeters]

🦟 Bug fix

Part of gazebo-tooling/release-tools#862.

Summary

Version v2 of the actions/checkout workflow is deprecated, so switch to v3.

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 #85 (f67f1b2) into ign-utils1 (ec24c32) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           ign-utils1      #85   +/-   ##
===========================================
  Coverage       91.97%   91.97%           
===========================================
  Files               6        6           
  Lines             137      137           
===========================================
  Hits              126      126           
  Misses             11       11           

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