[ReleasePR alertsmanagement] [Hub Generated] Review request for Microsoft.AlertsManagement to add version stable/2021-08-08 (#2116)

Create to sync Azure/azure-rest-api-specs#17162
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/main...AzureSDKAutomation:sdkAuto/alertsmanagement?expand=1)

Author: msftbot[bot]