Yesterday, Microsoft announced the managed disks support for all new created Azure DevTest Lab labs.
-> https://azure.microsoft.com/en-us/updates/azure-devtest-labs-managed-disks-supported-in-new-labs/
For existing DevTest Lab labs have a look at my previous blog post to realize the use of managed disks.
-> https://www.danielstechblog.io/using-managed-disks-azure-devtest-lab/