Document new variable PSApplicationOutputEncoding

[jborean93]

PR Summary

Documents the new variable $PSApplicationOutputEncoding which is used to control the encoding PowerShell uses when reading output from an external application.

This also removes the example for $OutputEncoding as it is wrong. The $OutputEncoding variable is only used as the encoding that PowerShell writes data through the external application's stdin pipe. The command line is always a string on Windows/Linux/macOS so is unaffected by this option.

• See Add PSApplicationOutputEncoding variable PowerShell/PowerShell#21219 for the PR that is attempting to add this feature.

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
• Contributor's Guide: I have read the contributors guide.
• Style: This PR adheres to the style guide.

[jborean93]

Depending on what happens with #10877, I might just revert the changes to $OutputEncoding here in favour of that PR and keep this focused on the new proposed variable.

Edit: PR has been merged so I've rebased the changes here.

[learn-build-service-prod]

Learn Build status updates of commit 823964e:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/7.5/Microsoft.PowerShell.Core/About/about_Character_Encoding.md	✅Succeeded	View (powershell-7.5)
reference/7.5/Microsoft.PowerShell.Core/About/about_Preference_Variables.md	✅Succeeded	View (powershell-7.5)

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