add new version 2021-09-01-preview for signalr#15864

jianyexi merged 4 commits intoAzure:mainAzure/azure-rest-api-specs:mainfrom

wanlwanl:wanl/signalr-09-01-previewwanlwanl/azure-rest-api-specs:wanl/signalr-09-01-previewCopy head branch name to clipboard