chore: add mrgrain to mergify as core contributor (#21031)

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.mergify.yml

@@ -10,7 +10,7 @@ pull_request_rules:
  label:
  add: [ contribution/core ]
  conditions:
- - author~=^(RomainMuller|garnaat|skinny85|rix0rrr|NGL321|Jerry-AWS|MrArnoldPalmer|iliapolo|pkandasamy91|SoManyHs|uttarasridhar|otaviomacedo|madeline-k|kaizencc|comcalvi|Chriscbr|corymhall|peterwoodworth|ryparker|TheRealAmazonKendra|yuth|vinayak-kukreja)$
+ - author~=^(RomainMuller|rix0rrr|Jerry-AWS|MrArnoldPalmer|iliapolo|pkandasamy91|SoManyHs|uttarasridhar|otaviomacedo|madeline-k|kaizencc|comcalvi|corymhall|peterwoodworth|ryparker|TheRealAmazonKendra|yuth|vinayak-kukreja|mrgrain)$
  - -label~="contribution/core"
  - name: automatic merge
  actions: