PR Summary

Experimental feature list is corrupted in powershell.config.json in some OS packages (e.g. PowerShell-7.2.0-preview.3-win-x64.zip and PowerShell-7.2.0-preview.3-win-x64.msi and nightly builds).
This results in all experimental features disabled by default (unless overridden by another-scoped config).
This PR fixes experimental feature list in powershell.config.json.

Problem screenshot:

PR Context

While fixing another issue, #14748 changed the type of collection with experimental features from System.Array to System.Collections.ArrayList.
The later used the ArrayList.Add(Object) method returns an index at which the value has been added, which ends up in the output stream, contaminating the expected output with experimental features.

PR Checklist

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  • Experimental feature(s) needed
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• User-facing changes
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  • Documentation needed
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