chore(buildpacks): update heroku-buildpack-grails to v21

Author: Matthew Fisher

rootfs/builder/install-buildpacks

@@ -33,7 +33,7 @@ download_buildpack https://github.com/heroku/heroku-buildpack-ruby.git
 download_buildpack https://github.com/heroku/heroku-buildpack-nodejs.git         v91
 download_buildpack https://github.com/heroku/heroku-buildpack-java.git           v48
 download_buildpack https://github.com/heroku/heroku-buildpack-gradle.git         v18
-download_buildpack https://github.com/heroku/heroku-buildpack-grails.git         v20
+download_buildpack https://github.com/heroku/heroku-buildpack-grails.git         v21
 download_buildpack https://github.com/heroku/heroku-buildpack-play.git           v26
 download_buildpack https://github.com/heroku/heroku-buildpack-python.git         v85
 download_buildpack https://github.com/heroku/heroku-buildpack-php.git            v114