fix: accept decomposed DBs in PostgreSQL config validation

What does this MR do?

fix: accept decomposed DBs in PostgreSQL config validation

Since 19.0, the GitLab chart requires an externally configured PostgreSQL database and validates this during configuration checks.

The validation should also accept decomposed database setups. While no longer documented, they remain supported for backward compatibility.

Related MR: !2122 (merged)

Closes #6521 (closed)

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