Update APT repo for Bionic Beaver (Ubuntu 18.04)

[psyraxaus]

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Describe the issue

Unable to install zen using the APT repo on Ubuntu 18.04 (bionic beaver)

Can you reliably reproduce the issue?

If so, please list the steps to reproduce below:

1. Trying to install from the APT repo using echo 'deb https://zencashofficial.github.io/repo/ '$(lsb_release -cs)' main' | sudo tee --append /etc/apt/sources.list.d/zen.list
2. Results in Ign:4 https://zencashofficial.github.io/repo bionic InRelease Err:5 https://zencashofficial.github.io/repo bionic Release 404 Not Found [IP: 185.199.110.153 443] when running sudo apt-get update

Expected behaviour

Should install the zen from the APT repo

[Sto1cNate]

Need to update the /etc/apt/sources.list.d/zen.list file. Change from bionic to xenial. deb https://zencashofficial.github.io/repo/ xenial main #deb https://zencashofficial.github.io/repo/ bionic main