Revert database version from POSTGRES_17 to POSTGRES_15

[fredzqm]

Reverts #9211

[gemini-code-assist]

Summary of Changes

Hello @fredzqm, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request aims to roll back the default PostgreSQL database version used when provisioning new Cloud SQL instances. The change specifically targets the createInstance function, ensuring that new instances are created with POSTGRES_15 instead of the previously set POSTGRES_17, likely to maintain compatibility or stability with existing systems or supported versions.

Highlights

• Database Version Reversion: The pull request reverts the default PostgreSQL database version used for Cloud SQL instance creation from "POSTGRES_17" back to "POSTGRES_15".

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[gemini-code-assist]

Code Review

This pull request reverts the default database version for new Cloud SQL instances from POSTGRES_17 to POSTGRES_15. This is a straightforward change and seems reasonable, likely aimed at using a more stable and widely available database version. My main feedback is a suggestion to improve maintainability by extracting the hardcoded database version string into a constant. This would make future updates to this value easier and less error-prone.

[gemini-code-assist]

This hardcoded database version string was recently changed and is now being reverted, which indicates it might be a configuration value that changes over time. To improve maintainability and make future updates easier and safer, consider extracting "POSTGRES_15" into a named constant defined at the top of the file (e.g., const DEFAULT_DATABASE_VERSION = "POSTGRES_15";).

[yuchenshi]

Changelog please