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/appendices/0.20.0-migration-guide.md

@@ -2,6 +2,8 @@
 
 ## Removals
 
+### Command removals
+
 The following commands were previously deprecated and were removed in this release.
 
 - `apps`: Use `apps:list` instead.
@@ -26,6 +28,23 @@ The refactor of the proxy plugin removes the following functions from being sour
 
 The informal policy is to allow removal of functions within a `functions` file if they are not used in OSS plugins. Should you be affected by any such removals, please file an issue to expose the functionality via a plugin trigger.
 
+### Remove support for Unsupported Operating Systems
+
+Dokku will no longer distribute packages for Ubuntu versions that have reached either End of Life or End of Standard Support (for LTS releases). This currently includes the following releases:
+
+- Ubuntu 14.04 (trusty)
+- Ubuntu 14.10 (utopic)
+- Ubuntu 15.04 (vivid)
+- Ubuntu 15.10 (wily)
+- Ubuntu 16.10 (yakkety)
+- Ubuntu 17.04 (zesty)
+- Ubuntu 17.10 (artful)
+
+Dokku will no longer distribute packages for Debian versions that are not `stable` or `oldstable`. This currently includes the following releases:
+
+- Debian 7 (wheezy)
+- Debian 8 (jessie)
+
 ## Deprecations
 
 - `nginx:show-conf` has been deprecated in favor of `nginx:show-config`.