fix: Db subnets can use private route tables

[suminhong]

Description

database/redshift/elasticache subnets can use private route tables.

Motivation and Context

• "Error in function call" when using neither private subnets nor NAT #944

Breaking Changes

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

[github-actions]

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with no activity. Remove stale label or comment or this PR will be closed in 10 days

[github-actions]

This PR was automatically closed because of stale in 10 days

[github-actions]

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