Remove support for Heroku-16

[edmorley]

The Heroku-16 stack reached end-of-life on May 1st, 2021, and in a few days (June 1st, 2021) the Heroku-16 build system will be decommissioned, meaning no further builds will be possible:
https://help.heroku.com/0S5P41DC/heroku-16-end-of-life-faq

As such, support for Heroku-16 can shortly be dropped:
https://github.com/heroku/heroku-buildpack-google-chrome/search?q=16

There are also some Cedar-14 references that can be removed :-)