Remove 'All rights reserved' from the copyright headers

[xtqqczze]

Since July 2019 Microsoft no longer recommend the term All Rights Reserved to be in MIT license copyright headers.

PowerShell repo has already made these changes:
PowerShell/PowerShell#12190

We should update the headers to match the following :

// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.

See: https://docs.opensource.microsoft.com/content/releasing/copyright-headers.html
(link may require Microsoft credentials)