Remove support for Heroku-18

[edmorley]

Heroku-18 is EOL, so can't be used by customers any more.

As such the Heroku-18 pgbouncer binary, CI matrix entry, Makefile target, and README mentions could be removed:
https://github.com/search?q=repo%3Aheroku%2Fheroku-buildpack-pgbouncer%2018&type=code

[brahyt-sf]

#191