[DBMON-5975] Upgrade libpq to 18.3

[eric-weaver]

What does this PR do?

Upgrades the bundled libpq from 16.9 to 18.3 across all builder images (Linux x86_64, Linux aarch64, macOS, Windows).

On Windows, PostgreSQL 18 removed the legacy Perl MSVC build system (build.pl), so the build is migrated to Meson + Ninja. This includes:

• Installing winflexbison (flex/bison are required build prerequisites for PostgreSQL from source)
• Installing meson and ninja via pip
• Replacing the build.pl/install.pl commands with meson setup + ninja + ninja install
• Deleting the now-unused pg_config.pl override file

No integration code changes — libpq 18 is backward-compatible with all supported PostgreSQL server versions.

Motivation

We're currently behind multiple major versions that include security fixes and improvements and features

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[chatgpt-codex-connector]

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[dd-octo-sts]

⚠️ Recommendation: Add qa/skip-qa label

This PR does not modify any files shipped with the agent.

To help streamline the release process, please consider adding the qa/skip-qa label if these changes do not require QA testing.

[dd-octo-sts]

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