pipelines: document PostgreSQL as development-only for KFP 2.18

[kaikaila]

Summary

• Add KFP database configuration guidance.
• Document PostgreSQL as development/evaluation-only for KFP 2.18.
• Recommend MySQL for all production deployments.
• Link the MLMD PostgreSQL concurrency limitation in bug(backend): concurrent MLMD type creation can fail PostgreSQL pipeline runs in 2.18 pipelines#14353.
• Remove language implying PostgreSQL support will arrive in the next tagged release.

Validation

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[google-oss-prow]

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[kaikaila]