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

• [X] 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)
• [X] codecheck passed (See contributing)
• [X] 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[bot]]

Codecov Report

Merging #526 (647b35c) into ign-gui3 (3a87973) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 647b35c differs from pull request most recent head ac1f2a2. Consider uploading reports for the commit ac1f2a2 to get more accurate results

@@            Coverage Diff            @@
##           ign-gui3     #526   +/-   ##
=========================================
  Coverage     73.82%   73.82%           
=========================================
  Files            30       30           
  Lines          3270     3270           
=========================================
  Hits           2414     2414           
  Misses          856      856           

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