Renovate to nixica/horizon-advance shape (9.4.x)

Largest of the four lts renovations. Was the only branch still using horizon-core directly via a dead 'sts-945' ref (Bug years stale) plus a custom horizon-shell/lint-utils setup. Switches to the canonical nixica + horizon-advance shape, drops several legacy packages (pandoc, hls, minio-hs, net-mqtt, network-conduit-tls, postgres-options, wuss, req) that transitively required the deprecated 'connection' library, and prunes ~390 pkgs/*.nix files that duplicated horizon-advance. See commit message for full details.