Replace New-Object calls with ::new()

[fflaten]

PR Summary

Replaces $SafeCommands['New-Object'] calls with ::new() for improved readability and minor perf. Also removes a helper method in Pester.Factory that's no longer needed.

Leaving similar change in tests for a future PR to make sure any failures are actually code's fault.

PR Checklist

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• Tests are added/update (if required)
• Documentation is updated/added (if required)