feat: add OSShowcase field to ComputerPrestageSubsetSkipSetupItems (Jamf Pro 11.21)

[neilmartin83]

Change

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New Skip Setup Item for Computer PreStages. Introduced in Jamf Pro 11.21.

Type of Change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update (Wiki)
• Refactor (refactoring code, removing code, changing code structure)

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
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