chore: greet new PR contributors using mergeable #9434
                
     Merged
            
            
          
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Description
Replaces and closes #8885
Test run 1: paulbalandan/b40d520f57f086f2e58d1f21d093935d#6
Test run 2: paulbalandan/b40d520f57f086f2e58d1f21d093935d#7
API Ref: https://docs.github.com/en/webhooks/webhook-events-and-payloads?actionType=opened#pull_request
Checklist: