fix: remove exact version pin for libpq-dev in Dockerfile (#12808)

[OliverCostello1]

Summary

Closes #12808

Removes the exact version pin on libpq-dev in docker/Dockerfile. This pin has required 5+ manual bumps in ~18 months because Debian removes old package versions from apt repos whenever it publishes a security update, breaking Docker builds until a maintainer notices and bumps the pin.

What changed

docker/Dockerfile: libpq-dev=13.23-0+deb11u3 → libpq-dev (unpinned), with an explanatory comment.

Why this is safe

The apt-mark hold libpq-dev line that immediately follows already prevents apt-get dist-upgrade from changing the installed version during the build. The exact-version pin was providing no additional protection — it was only causing breakage when Debian published patch updates.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required or relevant for this PR
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.)

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