{Backup} Add mapping for eastus3 Cross region support by adding secondary region mapping (#32016)

Author: prog-rajkamal

src/azure-cli/azure/cli/command_modules/backup/custom.py

@@ -71,6 +71,7 @@
     "eastasia": "southeastasia",
     "eastus": "westus",
     "eastus2": "centralus",
+    "eastus3": "westus3",
     "eastus2euap": "centraluseuap",
     "francecentral": "francesouth",
     "francesouth": "francecentral",