feat: Update default Cloud SQL Postgres version to 17 and add more versions

[google-labs-jules]

In src/gcp/cloudsql/cloudsqladmin.ts, we currently default to postgres 15. This change updates that to default to postgres 17.

This also adds Postgres 16, 17, and 18 to the available databaseVersion types in src/gcp/cloudsql/types.ts and adds a changelog entry to CHANGELOG.md.

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PR created automatically by Jules for task 10648715265329793894

[google-labs-jules]

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[fredzqm]

#9279

Need to roll this back. Unfortunately, CSQL has a validation that prevents newer Postgres versions.