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 #490 (6174f61) into ign-common3 (3771b0d) will not change coverage.
The diff coverage is n/a.

❗ Current head 6174f61 differs from pull request most recent head 7b7accd. Consider uploading reports for the commit 7b7accd to get more accurate results

@@             Coverage Diff              @@
##           ign-common3     #490   +/-   ##
============================================
  Coverage        82.66%   82.66%           
============================================
  Files               72       72           
  Lines             9244     9244           
============================================
  Hits              7642     7642           
  Misses            1602     1602           

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