CodeGen from PR 15713 in Azure/azure-rest-api-specs

[Hub Generated] Public private branch 'dev-maintenance-Microsoft.Maintenance-2021-09-01-preview' (#15713)

* Adds base for updating Microsoft.Maintenance from version preview/2021-04-01-preview to version 2021-09-01-preview

* Updates readme

* Updates API version in new specs and examples

Author: SDKAuto