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/development/testing.md

@@ -9,11 +9,11 @@ We maintain the Dokku test harness within the `tests` directory:
 
 ## Continuous Integration
 
-All pull requests have tests run against them on [CircleCI](https://circleci.com/), a continuous integration platform that provides Docker support for Ubuntu Trusty 16.04.
+All pull requests have tests run against them on [Github Actions](https://github.com/features/actions), a continuous integration platform that provides Docker support for Ubuntu Trusty 18.04.
 
 If you wish to skip tests for a particular commit, e.g. documentation changes, you may add the `[ci skip]` designator to your commit message. Commits that _should_ be tested but have the above designator will not be merged.
 
-While we do provide official packages for a variety of platforms, as our test suite currently runs on Ubuntu Trusty 16.04, we only provide official installation support for that platform and the latest LTS release of Ubuntu (currently 18.04).
+While we do provide official packages for a variety of platforms, as our test suite currently runs on Ubuntu Trusty 18.04, we only provide official installation support for that platform and the latest LTS release of Ubuntu (currently 20.04).
 
 ## Local Test Execution
 

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.

docs/home.html

@@ -140,7 +140,7 @@
           <p class="line">
             <span class="path"></span>
             <span class="prompt">$</span>
-            <span class="command">echo "xenial bionic focal" | grep -q "$OS_ID" || OS_ID="bionic"</span>
+            <span class="command">echo "bionic focal" | grep -q "$OS_ID" || OS_ID="bionic"</span>
           </p>
           <p class="line">
             <span class="path"></span>