Add support for Heroku-22

[vseguin]

Hi!

Heroku-22 was just released and would be great to add support for these binaries.

I've tried to follow the process to recompile on heroku-22 based on the README (see fork here - https://github.com/vseguin/heroku-pdftk-buildpack) - but it seems it doesn't compile anymore... I get the following error 👇

2022-06-21T13:46:03.570942+00:00 app[web.1]: /app/.apt/usr/bin/../lib/gcc/x86_64-linux-gnu/6/jc1: error while loading shared libraries: libisl.so.19: cannot open shared object file: No such file or directory

Tried fiddling with the Aptfile but might be something to do with more recent Ubuntu versions?
Any pointer would be appreciated - happy to recompile and submit a PR if anybody has an idea how to get past that issue.

[FeeJai]

This one works: https://github.com/FeeJai/heroku-pdftk-buildpack

[stevenharman]

I've also built a pdftk-java version of the buildpack, which uses JAVA Native Images (so you don't need the full JVM. I'm published it to the Heroku Buildpack Registry, so you can add it like so:

$ heroku buildpacks:add --index=1 stevenharman/pdftk-java-native --app=<YOUR-APP-NAME>

My version of the buildpack also supports configuring different versions, and will cache the downloaded package between builds.