Update forum link

Author: PatAltimore

articles/blockchain/service/TOC.yml

@@ -58,7 +58,7 @@
   - name: Community
     href: https://techcommunity.microsoft.com/t5/Blockchain/bd-p/AzureBlockchain
   - name: Forum
-    href: https://social.msdn.microsoft.com/Forums/home?forum=azureblockchain
+    href: https://aka.ms/workbenchforum
   - name: Product feedback
     href: https://feedback.azure.com/forums/921130-azure-blockchain-service
   - name: Pricing