Remove support for building Heroku-18 (heroku#240)

Since Ubuntu 18.04 LTS and this Heroku-18 has reached end-of-life, and so
there will be no more security updates released upstream, and so no point
in us continuing to rebuild Heroku-18 images each time a stack image release
is performed.

See:
https://help.heroku.com/X5OE6BCA/heroku-18-end-of-life-faq

GUS-W-10446187.

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        stack-version: ["18", "20", "22"]
+        stack-version: ["20", "22"]
     steps:
       - name: Checkout
         uses: actions/checkout@v3

BUILD.md

@@ -7,15 +7,15 @@ https://johndjameson.com/blog/updating-your-shell-with-homebrew/
 
 ## Adding packages to the stack image
 
-Add the package you want to the appropriate `setup.sh` for example `heroku-18/setup.sh`:
+Add the package you want to the appropriate `setup.sh` for example `heroku-22/setup.sh`:
 
 ```diff
 +    libc6-dev \
 ```
 
 Once done, run the `bin/build.sh` locally to generate the corresponding `installed-packages.txt`.
 
-The `*-build` variants include all the packages from the non-build variant by default. This means that if you're adding a package to both, you only need to add them to the non-build variant. The example above will add `libc6-dev` to both `heroku-18` and `heroku-18-build`.
+The `*-build` variants include all the packages from the non-build variant by default. This means that if you're adding a package to both, you only need to add them to the non-build variant. The example above will add `libc6-dev` to both `heroku-22` and `heroku-22-build`.
 
 The `*cnb*` variants inherit the installed packages from the non-`*cnb*` variant. Add packages to a non-`*cnb*` variant to add them to the `*cnb*` variant.
 
@@ -33,7 +33,7 @@ If you're building on a machine with an architecture other than amd64, set `DOCK
 
     DOCKER_DEFAULT_PLATFORM=linux/amd64 ./bin/build.sh 22
 
-The supported stacks are: `18`, `20`, and `22`. This script will build a family
+The supported stacks are: `20` and `22`. This script will build a family
 of 4 images:
 
 * `heroku/heroku:{STACK_VERSION}` - The runtime stack image for the Heroku platform