feat: drop support for unsupported Debian and Ubuntu releases

While the latest packages may continue to work on other releases, we will no longer officially support these releases, nor will we distribute packages.

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 x64 - Any currently supported release](https://www.ubuntu.com/download), [Debian 8.2 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 16.04/18.04 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.
@@ -27,7 +27,7 @@ sudo DOKKU_TAG=v0.19.13 bash bootstrap.sh
 
 The installation process takes about 5-10 minutes, depending upon internet connection speed.
 
-If you're using Debian 8 or Ubuntu 14.04, make sure your package manager is configured to install a sufficiently recent version of nginx<sup>[3]</sup>, otherwise, the installation may fail due to `unmet dependencies` relating nginx.
+If you're using Debian 9 or Ubuntu 18.04, make sure your package manager is configured to install a sufficiently recent version of nginx<sup>[3]</sup>, otherwise, the installation may fail due to `unmet dependencies` relating nginx.
 
 #### 2. Setup SSH key and Virtualhost Settings