engine: remove ubuntu 16.04 "xenial" and add deprecation warning

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

engine/install/ubuntu.md

@@ -31,10 +31,18 @@ versions:
 - Ubuntu Groovy 20.10
 - Ubuntu Focal 20.04 (LTS)
 - Ubuntu Bionic 18.04 (LTS)
-- Ubuntu Xenial 16.04 (LTS)
 
 Docker Engine is supported on `x86_64` (or `amd64`), `armhf`, and `arm64` architectures.
 
+> Ubuntu 16.04 LTS "Xenial Xerus" end-of-life
+> 
+> Ubuntu Linux 16.04 LTS reached the end of its five-year LTS window on April
+> 30th 2021 and is no longer supported. Docker no longer releases packages for
+> this distribution (including patch- and security releases). Users running
+> Docker on Ubuntu 16.04 are recommended to update their system to a currently
+> supported LTS version of Ubuntu.
+{: .important }
+
 ### Uninstall old versions
 
 Older versions of Docker were called `docker`, `docker.io`, or `docker-engine`.