[ReleasePR mysql] ARM ID annotations for MySQL (#2515)

Create to sync Azure/azure-rest-api-specs#19594
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/main...azure-sdk:sdkAuto/mysql?expand=1)

Author: msftbot[bot]

schemas/2021-05-01/Microsoft.DBforMySQL.json

@@ -483,10 +483,12 @@
       "properties": {
         "delegatedSubnetResourceId": {
           "type": "string",
+          "format": "arm-id",
           "description": "Delegated subnet resource id used to setup vnet for a server."
         },
         "privateDnsZoneResourceId": {
           "type": "string",
+          "format": "arm-id",
           "description": "Private DNS zone resource id."
         }
       },