change main branch to feature/v4 in js sdk (Azure#15046)

Author: dw511214992

specificationRepositoryConfiguration.json

@@ -13,7 +13,8 @@
     },
     "azure-sdk-for-js": {
       "integrationRepository": "AzureSDKAutomation/azure-sdk-for-js",
-      "mainRepository": "Azure/azure-sdk-for-js"
+      "mainRepository": "Azure/azure-sdk-for-js",
+      "mainBranch": "feature/v4"
     },
     "azure-sdk-for-net": {
       "integrationRepository": "AzureSDKAutomation/azure-sdk-for-net",