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 #115 (b93a23c) into ign-plugin1 (67242cb) will not change coverage.
The diff coverage is n/a.

❗ Current head b93a23c differs from pull request most recent head 38ce13e. Consider uploading reports for the commit 38ce13e to get more accurate results

@@             Coverage Diff              @@
##           ign-plugin1     #115   +/-   ##
============================================
  Coverage        99.82%   99.82%           
============================================
  Files               15       15           
  Lines              584      584           
============================================
  Hits               583      583           
  Misses               1        1           

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