[zmstone]

Build images for emqx/otp 27.3.4.2-9 (tag OTP-27.3.4.2-9) and 28.4.1-5 (tag OTP-28.4.1-5).

Both releases add {packet, mqtt} support to ssl:setopts/2 (emqx/otp#110, emqx/otp#111), which EMQX needs to
start a TLS connection in {packet, raw} and switch to frame mode after CONNECT.

debian11 build fix

The debian11 jobs fail for every OTP version, on main as well as here:

E: Failed to fetch http://deb.debian.org/debian-security/pool/updates/main/g/glibc/libc-l10n_2.31-13+deb11u14_all.deb  404  Not Found

Bullseye is end of life. deb.debian.org still publishes the bullseye-security index, but its pool files are
gone, so apt resolves a package version it can no longer download. archive.debian.org carries bullseye and
bullseye-updates, but no bullseye security suite (its debian-security archive stops at buster).

So apt is pointed at archive.debian.org and the security line is dropped:

• in debian11/Dockerfile, for base image rebuilds;
• in the main Dockerfile, guarded by ID=debian VERSION_ID=11, because the published base image
  (base-5.0:debian11-*, built 2026-07) carries the old sources. That guard can go once the base images are
  rebuilt from this branch.

Verified in ghcr.io/emqx/emqx-builder/base-5.0:debian11-amd64: apt-get update is clean and locales
installs (2.31-13+deb11u11, the last version in the archive).

Note that debian11 images no longer receive bullseye security updates; the packages come from the archived
release. Worth revisiting if Debian later publishes bullseye on archive.debian.org/debian-security.

After merge, push tag 6.1-9 to build and publish the images.