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 #488 (7182864) into ign-physics2 (779e5ff) will not change coverage.
The diff coverage is n/a.

❗ Current head 7182864 differs from pull request most recent head 40f257d. Consider uploading reports for the commit 40f257d to get more accurate results

@@              Coverage Diff              @@
##           ign-physics2     #488   +/-   ##
=============================================
  Coverage         83.15%   83.15%           
=============================================
  Files               108      108           
  Lines              4210     4210           
=============================================
  Hits               3501     3501           
  Misses              709      709           

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