Make sql db instance recreate when going private -> public

[modular-magician]

fixes #2964 (comment)

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Release Note Template for Downstream PRs (will be copied)

sql: fixed an issue where `google_sql_database_instance` would throw an error when removing `private_network`. Removing `private_network` now recreates the resource.

Derived from GoogleCloudPlatform/magic-modules#3882

[ghost]

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