Update install notes for SxS with PS6

[sdwheeler]

No description provided.

[opbld32]

Docs Build status updates of commit dc281ea:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/install/Installing-PowerShell-Core-on-Linux.md	✅Succeeded	View (powershell-3.0) View (powershell-4.0) View (powershell-5.0) View (powershell-5.1) View (powershell-6) View (powershell-7) View (powershell-7.x)
reference/docs-conceptual/install/Installing-PowerShell-Core-on-Windows.md	✅Succeeded	View (powershell-3.0) View (powershell-4.0) View (powershell-5.0) View (powershell-5.1) View (powershell-6) View (powershell-7) View (powershell-7.x)
reference/docs-conceptual/install/Installing-PowerShell-Core-on-macOS.md	✅Succeeded	View (powershell-3.0) View (powershell-4.0) View (powershell-5.0) View (powershell-5.1) View (powershell-6) View (powershell-7) View (powershell-7.x)
reference/docs-conceptual/whats-new/What-s-New-in-PowerShell-70.md	✅Succeeded	View (powershell-3.0) View (powershell-4.0) View (powershell-5.0) View (powershell-5.1) View (powershell-6) View (powershell-7) View (powershell-7.x)

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[theJasonHelmick]

Looks good!