chore(QOV-1296): rename custom VPC private subnets fields

[benjaminch]

Ticket: QOV-1296

What does this PR do?

Link to the JIRA ticket

Put description here

Screenshot of the feature

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PR Checklist

• [ ] This PR introduces breaking change(s) and has been labeled as such
• [ ] This PR introduces new store changes
• [ ] I made sure the code is type safe (no any)

[RemiBonnet]

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