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 #391 (360716f) into ign-transport8 (559ba5a) will increase coverage by 0.01%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           ign-transport8     #391      +/-   ##
==================================================
+ Coverage           83.61%   83.63%   +0.01%     
==================================================
  Files                  51       51              
  Lines                5036     5036              
==================================================
+ Hits                 4211     4212       +1     
+ Misses                825      824       -1     

see 1 file with indirect coverage changes

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