Add Windows 10 LTSC 2021 GVLK

[CometstarMH]

https://docs.microsoft.com/en-us/windows-server/get-started/kms-client-activation-keys
The commit that "adds" the key to the Microsoft doc: MicrosoftDocs/windowsserverdocs@38d0343

"Add" is probably not the correct word, since the key is the same as the one for Windows 10 Enterprise LTSC 2019. Probably need to modify the DisplayName for LTSC 2019 in KmsDataBase.xml and Keys.md in the docs.