docs: Add PowerShell v6+ prerequisite for Windows users to Copilot CLI install guide

[aymanaboghonim]

Copilot Chat generated this pull request.

Prompt summary - submitted by @aymanaboghonim

Add a Windows-specific prerequisite for PowerShell v6+ to the Copilot CLI install guide, as required by the official documentation. No other changes were made.

Summary:
This PR surgically adds a single prerequisite line for Windows users to the Copilot CLI installation guide, ensuring the documentation matches the official requirements. No other content was changed.

Reference:
See the official Copilot CLI prerequisites: https://github.com/github/copilot-cli?tab=readme-ov-file#prerequisites

[Copilot]

Pull Request Overview

This PR adds a Windows-specific prerequisite for PowerShell version 6 or later to the Copilot CLI installation guide, aligning the documentation with the official requirements.

• Added PowerShell v6+ prerequisite for Windows users in the prerequisites list

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Source	Review	Production	What Changed
copilot/how-tos/set-up/install-copilot-cli.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Sharra-writes]

@aymanaboghonim Our documentation for Windows is often lacking, so I'll definitely look into this.