[EventHubs] update cperf core baseeline (Azure#38952)

Author: swathipil

sdk/eventhub/perf-tests.yml

@@ -5,7 +5,7 @@ Project: sdk/eventhub/azure-eventhub
 PrimaryPackage: azure-eventhub
 
 PackageVersions:
-- azure-core: 1.26.2
+- azure-core: 1.31.0
   azure-eventhub: 5.12.1
 - azure-core: source
   azure-eventhub: source