Add support for PowerShell 7.

[alekdavis]

PowerShell 7 has some breaking changes (e.g. these), so need to add fixes to allow the script to work with PowerShell 7.x. In particular, need to modify the GetPowerShellVersion method to accommodate different version properties: Major, Minor, Patch, PreReleaseLabel, BuildLabel
(the last two may be null). There may be more issues.