CI workflow: use checkout v3 (#490)

[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 #213 (05ac985) into ign-launch2 (0dbd617) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 05ac985 differs from pull request most recent head 8924667. Consider uploading reports for the commit 8924667 to get more accurate results

@@             Coverage Diff              @@
##           ign-launch2     #213   +/-   ##
============================================
  Coverage        56.74%   56.74%           
============================================
  Files                2        2           
  Lines              326      326           
============================================
  Hits               185      185           
  Misses             141      141           

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