fix: preserve existing owner contributions during transition

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: Transitioned repos remove some existing contributions #2063
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

To work around all-contributors/cli#371, any existing owner contributions are merged with the default starters.

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (86ea767) to head (7b59fac).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2065   +/-   ##
=======================================
  Coverage   99.81%   99.82%           
=======================================
  Files         120      120           
  Lines        4999     5012   +13     
  Branches      313      316    +3     
=======================================
+ Hits         4990     5003   +13     
  Misses          9        9           

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[github-actions]

🎉 This is included in version v2.18.2 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀