chore: drop support for Ubuntu 16.04

As of April 2021, it will no longer be an LTS release, and thus us supporting it will increase maintenance burdens.

Also switch CI to use 18.04, so as to test what we currently support.

Closes #4505

docs/getting-started/install/debian.md

@@ -13,7 +13,7 @@ wget -nv -O - https://get.docker.com/ | sh
 # install dokku
 wget -nv -O - https://packagecloud.io/dokku/dokku/gpgkey | apt-key add -
 OS_ID="$(lsb_release -cs 2>/dev/null || echo "bionic")"
-echo "xenial bionic focal" | grep -q "$OS_ID" || OS_ID="bionic"
+echo "bionic focal" | grep -q "$OS_ID" || OS_ID="bionic"
 echo "deb https://packagecloud.io/dokku/dokku/ubuntu/ ${OS_ID} main" | sudo tee /etc/apt/sources.list.d/dokku.list
 sudo apt-get update -qq >/dev/null
 sudo apt-get -qq -y install dokku

docs/getting-started/installation.md

@@ -6,7 +6,7 @@ Dokku is an extensible, open source Platform as a Service that runs on a single
 
 To start using Dokku, you'll need a system that meets the following minimum requirements:
 
-- A fresh installation of [Ubuntu 16.04/18.04/20.04 x64](https://www.ubuntu.com/download), [Debian 9+ x64](https://www.debian.org/distrib/) or [CentOS 7 x64](https://www.centos.org/download/) *(experimental)* with the FQDN set <sup>[1]</sup>
+- A fresh installation of [Ubuntu 18.04/20.04 x64](https://www.ubuntu.com/download), [Debian 9+ x64](https://www.debian.org/distrib/) or [CentOS 7 x64](https://www.centos.org/download/) *(experimental)* with the FQDN set <sup>[1]</sup>
 - At least 1 GB of system memory <sup>[2]</sup>
 
 You can *optionally* have a domain name pointed at the host's IP, though this is not necessary.