Consider adding a higher version of PowerShell as a pre-requisite

[ryan-moreno]

In the README for the qsharp-runtime repo (https://github.com/microsoft/qsharp-runtime#readme), add a note that to the section "2. Run bootstrap.ps1 from PowerShell" explaining that this script needs to be run from a version of PowerShell >= 6.0

When running bootstrap.ps1 from the PowerShell that comes with vs code, this leads to the error:

.\prerequisites.ps1 : The script 'prerequisites.ps1' cannot be run because it contained a "#requires" statement for Windows PowerShell 6.0. The version of Windows PowerShell that is required by the script does not match the currently running version of Windows PowerShell

It would be even better if PowerShell 7 was installed via a set up script in the repository.