Fixes #11258 - Fix consistency of case for boolean variables

[sdwheeler]

PR Summary

Fix consistency of case for boolean variables

• Fixes AB#284425
• Fixes The $IsWindows variable of about_Automatic_Variables documentation suggests value is all Caps. Running "$IsWindows" in pwsh 7.4 shell proves otherwise. #11258

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.
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[learn-build-service-prod]

Learn Build status updates of commit 01a16c7:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/5.1/Microsoft.PowerShell.Core/About/about_Automatic_Variables.md	✅Succeeded	View (powershell-5.1)
reference/7.2/Microsoft.PowerShell.Core/About/about_Automatic_Variables.md	✅Succeeded	View (powershell-7.2)
reference/7.4/Microsoft.PowerShell.Core/About/about_Automatic_Variables.md	✅Succeeded	View (powershell-7.4)
reference/7.5/Microsoft.PowerShell.Core/About/about_Automatic_Variables.md	✅Succeeded	View (powershell-7.5)

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