remove istio exception

Package Owner Merge Request

Package Changes

What package changes are included with this merge request? The more detail here the better!

• Removal of istio-system from the excludedNamespaces for allowedDockerRegistries violations in chart/templates/gatekeeper/values.yaml
• Added notes about allowing the exception for development purposes with k3d

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Additional Details

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Known issues or expected conflicts?

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Closes #590

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