Fix New-TemporaryFile online help URI - #6608

This like resolves to PowerShell 5.1 (https://docs.microsoft.com/en-us/powershell/module/Microsoft.PowerShell.Utility/New-TemporaryFile?view=powershell-5.1) There is a PowerShell 6 page for this cmdlet (https://docs.microsoft.com/en-us/powershell/module/Microsoft.PowerShell.Utility/New-TemporaryFile?view=powershell-6)

For most of our cmdlets we default to 5.1. Joey Aiello (@joeyaiello) Are we going to change the default to 6?

So the new fwlink points to 5.1, and IMO it should probably point to the version-agnostic link (for which we can change the default later). This way, when we update to 6, we can rev the default for all modules on docs.microsoft.com to 6.1, and then we don't have to change the fwlinks.

However, if we go that route, we need to make sure that 6 has distinct fwlinks from 5.1, and i'm not sure we've got that work fully understood or costed (that's a lot of fwlinks).