Automatically configure `WEB_CONCURRENCY`

[edmorley]

The classic Python buildpack automatically configures the WEB_CONCURRENCY env var based on available total CPU and RAM:
https://github.com/heroku/heroku-buildpack-python/blob/main/vendor/WEB_CONCURRENCY.sh
https://devcenter.heroku.com/articles/python-concurrency

The env var is then used by web servers like gunicorn and uvicorn to automatically set the number of worker processes to spawn.

We should add something similar to the CNB.

GUS-W-9538302.