Deprecate beta CNB support

CNB support was introduced into this buildpack as an experiment. The experiment is over, anyone wanting to use the official CNB (still in "preview" support) should use https://github.com/heroku/buildpacks-ruby

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Deprecate CNB support, CNB users should use [heroku/buildpacks-ruby](https://github.com/heroku/buildpacks-ruby) instead (https://github.com/heroku/heroku-buildpack-ruby/pull/1445)
 
 ## [v268] - 2024-04-17
 

bin/build

@@ -8,10 +8,18 @@ APP_DIR=$(pwd)
 BIN_DIR=$(cd $(dirname $0); pwd)
 BUILDPACK_DIR=$(dirname $BIN_DIR)
 
-# legacy buildpack uses $STACK 
+# legacy buildpack uses $STACK
 export STACK=$CNB_STACK_ID
 
 source "$BIN_DIR/support/bash_functions.sh"
 heroku_buildpack_ruby_install_ruby "$BIN_DIR" "$BUILDPACK_DIR"
 
+cat<<EOF 1>&2
+    CNB no longer maintained
+
+    The heroku/heroku-ruby-buildpack previously implemented the Cloud Native Buildpack \(CNB\) spec beta support, however this functionality is no longer supported or maintained.
+
+    The Heroku Ruby CNB is now at https://github.com/heroku/buildpacks-ruby. See https://github.com/heroku/buildpacks for more information on using the Heroku CNB builder. To avoid interruptions switch to the new CNB as soon as possible.
+EOF
+
 $heroku_buildpack_ruby_dir/bin/ruby $BIN_DIR/support/ruby_build $APP_DIR $LAYERS_DIR $PLATFORM_DIR $PLAN