CI workflow: use checkout v3 (jammy)

[scpeters]

🦟 Bug fix

Part of gazebo-tooling/release-tools#862.

Summary

Version v2 of the actions/checkout workflow is deprecated, so switch to v3.

Follow-up to #526 that missed jammy when merged forward.

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 #536 (e981ecb) into ign-gui6 (b3903b0) will decrease coverage by 0.04%.
The diff coverage is n/a.

❗ Current head e981ecb differs from pull request most recent head 790378f. Consider uploading reports for the commit 790378f to get more accurate results

@@             Coverage Diff              @@
##           ign-gui6     #536      +/-   ##
============================================
- Coverage     69.83%   69.79%   -0.04%     
============================================
  Files            40       40              
  Lines          5324     5324              
============================================
- Hits           3718     3716       -2     
- Misses         1606     1608       +2     

see 1 file with indirect coverage changes

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