Explicitly define powershell dependency

Arcade build scripts run on whatever version of Powershell is available on the machine. Today that means we need to support Powershell v3.0, which leads to required changes like https://github.com/dotnet/arcade/pull/42.

As @natemcmaster suggested, we should be explicit about the minimum supported Powershell version that we've tested (even if that's back to 3.0).

> That makes sense. Maybe we should add `#requires`
```ps1
#requires -version 5.0
```

> https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_requires