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 #834 (6ddbf46) into ign-rendering3 (8c69375) will not change coverage.
The diff coverage is n/a.

❗ Current head 6ddbf46 differs from pull request most recent head d69c737. Consider uploading reports for the commit d69c737 to get more accurate results

@@               Coverage Diff               @@
##           ign-rendering3     #834   +/-   ##
===============================================
  Coverage           83.36%   83.36%           
===============================================
  Files                 100      100           
  Lines                7815     7815           
===============================================
  Hits                 6515     6515           
  Misses               1300     1300           

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