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 #331 (ef8402f) into ign-msgs5 (37fe037) will not change coverage.
The diff coverage is n/a.

❗ Current head ef8402f differs from pull request most recent head be5357c. Consider uploading reports for the commit be5357c to get more accurate results

@@            Coverage Diff             @@
##           ign-msgs5     #331   +/-   ##
==========================================
  Coverage      97.12%   97.12%           
==========================================
  Files              9        9           
  Lines            905      905           
==========================================
  Hits             879      879           
  Misses            26       26           

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