[ReleasePR eventgrid] Event Grid: add support for 2021-10-15-preview API version (#2138)

Create to sync Azure/azure-rest-api-specs#18349
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/main...AzureSDKAutomation:sdkAuto/eventgrid?expand=1)

Author: msftbot[bot]

schemas/2018-05-01/subscriptionDeploymentTemplate.json

@@ -884,6 +884,9 @@
             {
               "$ref": "https://schema.management.azure.com/schemas/2021-06-01-preview/Microsoft.EventGrid.json#/unknown_resourceDefinitions/eventSubscriptions"
             },
+            {
+              "$ref": "https://schema.management.azure.com/schemas/2021-10-15-preview/Microsoft.EventGrid.json#/unknown_resourceDefinitions/eventSubscriptions"
+            },
             {
               "$ref": "https://schema.management.azure.com/schemas/2021-12-01/Microsoft.EventGrid.json#/unknown_resourceDefinitions/eventSubscriptions"
             },