Update the Heroku-18 status to End-of-life (heroku#239)

Since the Heroku-18 stack reached end-of-life on April 30th, 2023:
https://devcenter.heroku.com/changelog-items/2583
https://help.heroku.com/X5OE6BCA/heroku-18-end-of-life-faq

The Dockerfiles/scripts for Heroku-18 have been left in the repo for
now, since Canonical will still be releasing an update or two
during the month of May.

GUS-W-10446187.

README.md

@@ -5,20 +5,20 @@
 This repository holds recipes for building [Heroku stack images](https://devcenter.heroku.com/articles/stack).
 The recipes are also rendered into Docker images that are available on Docker Hub:
 
-| Image                                     | Base                                  | Type                       | Status     |
-|-------------------------------------------|---------------------------------------|----------------------------|------------|
-| [heroku/heroku:18][heroku-tags]           | [ubuntu:18.04][ubuntu-tags]           | Heroku Runtime Stack Image | Deprecated |
-| [heroku/heroku:18-build][heroku-tags]     | [heroku/heroku:18][heroku-tags]       | Heroku Build Stack Image   | Deprecated |
-| [heroku/heroku:18-cnb][heroku-tags]       | [heroku/heroku:18][heroku-tags]       | CNB Runtime Stack Image    | Deprecated |
-| [heroku/heroku:18-cnb-build][heroku-tags] | [heroku/heroku:18-build][heroku-tags] | CNB Build Stack Image      | Deprecated |
-| [heroku/heroku:20][heroku-tags]           | [ubuntu:20.04][ubuntu-tags]           | Heroku Runtime Stack Image | Available  |
-| [heroku/heroku:20-build][heroku-tags]     | [heroku/heroku:20][heroku-tags]       | Heroku Build Stack Image   | Available  |
-| [heroku/heroku:20-cnb][heroku-tags]       | [heroku/heroku:20][heroku-tags]       | CNB Runtime Stack Image    | Available  |
-| [heroku/heroku:20-cnb-build][heroku-tags] | [heroku/heroku:20-build][heroku-tags] | CNB Build Stack Image      | Available  |
-| [heroku/heroku:22][heroku-tags]           | [ubuntu:22.04][ubuntu-tags]           | Heroku Runtime Stack Image | Suggested  |
-| [heroku/heroku:22-build][heroku-tags]     | [heroku/heroku:22][heroku-tags]       | Heroku Build Stack Image   | Suggested  |
-| [heroku/heroku:22-cnb][heroku-tags]       | [heroku/heroku:22][heroku-tags]       | CNB Runtime Stack Image    | Suggested  |
-| [heroku/heroku:22-cnb-build][heroku-tags] | [heroku/heroku:22-build][heroku-tags] | CNB Build Stack Image      | Suggested  |
+| Image                                     | Base                                  | Type                       | Status      |
+|-------------------------------------------|---------------------------------------|----------------------------|-------------|
+| [heroku/heroku:18][heroku-tags]           | [ubuntu:18.04][ubuntu-tags]           | Heroku Runtime Stack Image | End-of-life |
+| [heroku/heroku:18-build][heroku-tags]     | [heroku/heroku:18][heroku-tags]       | Heroku Build Stack Image   | End-of-life |
+| [heroku/heroku:18-cnb][heroku-tags]       | [heroku/heroku:18][heroku-tags]       | CNB Runtime Stack Image    | End-of-life |
+| [heroku/heroku:18-cnb-build][heroku-tags] | [heroku/heroku:18-build][heroku-tags] | CNB Build Stack Image      | End-of-life |
+| [heroku/heroku:20][heroku-tags]           | [ubuntu:20.04][ubuntu-tags]           | Heroku Runtime Stack Image | Available   |
+| [heroku/heroku:20-build][heroku-tags]     | [heroku/heroku:20][heroku-tags]       | Heroku Build Stack Image   | Available   |
+| [heroku/heroku:20-cnb][heroku-tags]       | [heroku/heroku:20][heroku-tags]       | CNB Runtime Stack Image    | Available   |
+| [heroku/heroku:20-cnb-build][heroku-tags] | [heroku/heroku:20-build][heroku-tags] | CNB Build Stack Image      | Available   |
+| [heroku/heroku:22][heroku-tags]           | [ubuntu:22.04][ubuntu-tags]           | Heroku Runtime Stack Image | Recommended |
+| [heroku/heroku:22-build][heroku-tags]     | [heroku/heroku:22][heroku-tags]       | Heroku Build Stack Image   | Recommended |
+| [heroku/heroku:22-cnb][heroku-tags]       | [heroku/heroku:22][heroku-tags]       | CNB Runtime Stack Image    | Recommended |
+| [heroku/heroku:22-cnb-build][heroku-tags] | [heroku/heroku:22-build][heroku-tags] | CNB Build Stack Image      | Recommended |
 
 ### Learn more