Refs #37010 - Ensure correct yaml for netplan

The introduced safe operator returns nil instead of false, but the
snippet expects true/false.

Refs #37034 - Fix ERB syntax for comments

Co-authored-by: Markus Bucher <bucher@atix.de>

Refs #37034 - Use apt-key add on Ubuntu 16.04

• On Ubuntu 16.04, which is supported by Canonical until 2026, you
  cannot place a GPG public key in "/etc/apt/trusted.gpg.d/" and use it
  to verify the signature of "Release" files.
• EoL Ubuntu 16.04: https://ubuntu.com/16-04...

Fixes #37011 - Apply latest Ubuntu Autoinstall userdata changes

• Add explicit fallback for offline installation
• Replace deprecated lock-passwd with lock_passwd
• Remove empty ssh keys array if none provided

Fixes #37193 - trap ERR not supported on dash

Fixes #36502 - iPXE Discovery Only Works on net0

Fixes #37189 - Registration before & after snippets

before_registration & after_registration snippets allow
users to add their code to the template without directly editing it.

Fixes #37142 - Cloud-init breaks due to newline

This fixes cloud-init based deployment to Proxmox for Debian, Ubuntu,
and SUSE Linux Enterprise Server. Previously, deployments failed due to
a newline in the echo command.

Fixes #37034 - Replace apt-key on Debian/Ubuntu

Fixes #37010 - Support safe navigation operator in safemode

Safemode 1.4.0 has added support for this. This makes sure that version
is included.