Modify and remove Heroku-specific code and documentation

[mathjazz]

We'll be migrating https://pontoon.mozilla.org/ and https://pontoon.allizom.org/ depoyments off of Heroku.

We can keep the following Heroku-specific code to keep Heroku as an unofficial deployment environment:
https://github.com/mozilla/pontoon/blob/main/.github/workflows/heroku.yml
https://github.com/mozilla/pontoon/blob/main/app.json
https://github.com/mozilla/pontoon/blob/main/requirements.txt
https://github.com/mozilla/pontoon/blob/main/bin/post_compile
https://github.com/mozilla/pontoon/blob/main/bin/heroku_postdeploy
https://github.com/mozilla/pontoon/blob/main/pontoon/base/management/commands/heroku_deploy_setup.py
https://github.com/mozilla/pontoon/blob/main/pontoon/settings/base.py#L921

We can also keep Heroku Deployment and Maintenance docs, but only as an alternative to #3842:
https://github.com/mozilla/pontoon/blob/main/docs/admin/deployment.rst
https://github.com/mozilla/pontoon/blob/main/docs/admin/maintenance.rst
https://github.com/mozilla/pontoon/blob/main/README.md?plain=1#L12-L19
https://github.com/mozilla/pontoon/blob/main/translate/README.md?plain=1#L103

This can be removed:
https://github.com/mozilla/pontoon/blob/main/pontoon/settings/base.py#L39
https://github.com/mozilla/pontoon/blob/main/package.json#L22