Use poppler-utils from stack

[dzuelke]

Current main and update-ffmpeg will now fail, as the recent stack image update brought in all the libraries that poppler-utils needs as part of the libvips install.

As a result, the find for .so files piped to xargs dirname in bin/compile will fail, because no .so files are actually added:

remote: -----> ActiveStorage Preview app detected
remote: -----> Installing binary dependencies for ActiveStorage Preview
remote:        Detected ffmpeg version change, flushing cache
remote:        Downloading packagesW: http://apt.postgresql.org/pub/repos/apt/dists/jammy-pgdg/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
remote: ..
remote:        Installing packages..
remote: dirname: missing operand
remote: Try 'dirname --help' for more information.
remote:  !     Push rejected, failed to compile ActiveStorage Preview app.
remote: 
remote:  !     Push failed

This simplifies bin/compile greatly by removing all the now-unnecessary Apt trickery:

% heroku buildpacks:set -i 1 https://github.com/heroku/heroku-buildpack-activestorage-preview.git#stack-poppler
Buildpack set. Next release on blooming-castle-09486 will use:
  1. https://github.com/heroku/heroku-buildpack-activestorage-preview.git#stack-poppler
  2. heroku/ruby
Run git push heroku main to create a new release using these buildpacks.

% git push heroku master
…
remote: -----> ActiveStorage Preview app detected
remote: -----> Installing binary dependencies for ActiveStorage Preview
remote:        Downloading ffmpeg version 5.1.2
remote:        Installing ffmpeg
remote:        Done
remote:        Exporting environment variables
remote:        Done
…

% heroku run which ffmpeg pdftocairo
Running which ffmpeg pdftocairo on ⬢ blooming-castle-09486... up, run.3697 (Basic)
/app/.heroku/activestorage-preview/bin/ffmpeg
/usr/bin/pdftocairo

GUS-W-12618598

Also closes #21

GUS-W-8276306

[dzuelke]

(CI will likely fail until rollout of https://devcenter.heroku.com/changelog-items/2549 has propagated)

[dzuelke]

Rebased on top of update-ffmpeg.

Once ready, please merge this after #31, and using a regular merge commit to ease merging/rebasing of stacked PR #34