Validate docs/quickstart.md syslog-ng snippet against syslog-ng v4

[azurebeard]

Context

docs/quickstart.md § 5b shows a syslog-ng destination snippet. The syntax was written against syslog-ng v3 conventions. syslog-ng v4 (released 2023) introduced some changes — notably tighter network() driver semantics — and operators on a current LTS distro may be running v4 by default.

Scope

Stand up a recent syslog-ng (v4.x) on a test VM, apply the snippet from docs/quickstart.md § 5b verbatim, send a couple of test events, confirm Harbinger sees them on 127.0.0.1:1514.

If the snippet works as-is on v4 — add a one-line note to that section confirming it's been validated against syslog-ng 4.x.

If the snippet needs tweaks on v4 — update the snippet, note the minimum version it applies to, and (if there's a v3-only version that differs) add a short "for syslog-ng v3" subsection.

Optionally validate against the latest Debian Stable and Ubuntu LTS syslog-ng packages and note the versions checked.

Acceptance criteria

• docs/quickstart.md § 5b snippet validated against at least one current syslog-ng release.
• Doc updated either with a "validated against vX.Y" note or with the corrected snippet plus an explanation.
• No regression for operators on the older syntax (don't break the v3 path silently).

Why this is a good first issue

Operations practice, not Python. Reasonable scope (1–2 hours of VM time). Result is concrete and verifiable.