docs: modernize the pg_durable release prompt (#246)

The previous version of the prompt predated most of the release
automation that has landed since it was first written — there's now CI
gating, Debian .deb packaging + GitHub Release automation, and
Docker/GHCR image publishing — and the old prompt described a much
simpler, mostly-manual flow. This brings it back in sync with reality.

Author: pinodeca