Add support for Windows Server 2016

What does this MR do?

This allows GitLab runner to run on Windows Server 2016.

Why was this MR needed?

GitLab Runner currently only supports Windows 1803 and 1809. Windows Server 2016 and 2019 are pretty common, but currently are not supported.

Are there points in the code the reviewer needs to double check?

Is the use of USER ContainerAdministrator a hard requirement? Using it breaks the build, removing it seems to make everything work. Also, based on some documentation I read the default user is ContainerAdministrator anyway:

Windows Server containers using process isolation are slightly different because they use the process identity within the container to access data, meaning that file ACLs are honored. The identity of the process running in the container ("ContainerAdministrator" on Windows Server Core and "ContainerUser" on Nano Server containers, by default)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

There is no directly related issue, but #4396 (closed) is somewhat related.