[ReleasePR synapse] [Hub Generated] Review request for Microsoft.Synapse to add version preview/2021-04-01-preview (#1587)

Create to sync Azure/azure-rest-api-specs#13874
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/master...AzureSDKAutomation:sdkAuto/synapse?expand=1)

Author: msftbot[bot]