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:

heroku-buildpack-pgbouncer/Makefile

Lines 3 to 5 in b229604

	build-heroku-16:
	@echo "Building pgbouncer in Docker for heroku-16..."
	@docker run -v $(shell pwd):/buildpack --rm -it -e "STACK=heroku-16" -w /buildpack heroku/heroku:16-build support/pgbouncer-build

https://github.com/heroku/heroku-buildpack-pgbouncer/blob/main/pgbouncer-heroku-16.tgz

[beanieboi]

closed by #151