Fix broken first-timers welcome link

[s4daexe]

Fixes #11877

This PR updates the broken First Timers Welcome link in CONTRIBUTING.md.

The previous link to https://code.publiclab.org/#r=all no longer resolves, so I replaced it with a working GitHub issues search link for open first-timers-only issues in this repository.

Checklist:

• PR is descriptively titled and links the original issue above
• Code is in uniquely-named feature branch and has no merge conflicts
• This is a documentation-only change, so no screenshots/GIFs are needed

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[s4daexe]

Hi! I checked the failing jobs and they seem unrelated to this documentation-only change.

• The functional tests failed because the cache service was unavailable.
• The integration/unit test jobs appear to fail during the Codecov upload step due to a rate limit/token issue.
• The system/comment-system tests fail during setup because actions/upload-artifact: v2 is deprecated.

Please let me know if there is anything I should update on my side. Thank you!