Remove support for the CMD shell for Windows from GitLab Runner

Overview

In GitLab 11.11, we announced the deprecation of the Windows Batch executor, cmd shell, for the GitLab Runner in favor of PowerShell.

Now we are planning to fully remove the cmd shell from GitLab Runner.

Plan

1. Announce the removal in an upcoming release post - no later than 16.1
2. Ensure that Powershell is the default shell for the Windows executor.
3. Fully remove cmd from Runner. Current target is 17.0

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