Fixed the regex of the target branch

[sima-zhu]

Fixed the failure: https://dev.azure.com/azure-sdk/public/_build/results?buildId=1360045&view=logs&j=90227089-9f51-5ecc-566c-84e1f2918394&t=0bfbd951-3ae9-5af7-a8d0-fd7a58328df7

Testing the changes:

1. Creating a testing PR to check the changes azure-sdk-for-java#27022
2. Testing for my changes azure-sdk-for-net#26945

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