Werk #13852: Win-agent installer creates allow-pull-mode file correctly

Previously, custom action in MSI failed if the available Powershell was
too old. Such failure made impossible the obtaining of the previously installed version of win-agent. For the use case update from 2.0 on 2.1, this means lack of connection between agent and site, i.e. stop of the monitoring.

Since this release, the uninstaller custom action supports also Powershell 4.0 thus solving the problem.

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