Decide whether to continue buildpack NLTK support

[edmorley]

The classic Python buildpack supports installing NLTK corpora via an nltk.txt file:
https://devcenter.heroku.com/articles/python-nltk
https://github.com/heroku/heroku-buildpack-python/blob/main/bin/steps/nltk
https://www.nltk.org/data.html

This is a someone infrequently used feature, and it seems it could be replaced by an inline buildpack that runs a single command.

Though we'd need to check whether NLTK_DATA needs to be set, or if we can rely on the default output location.

If we do decide to drop this feature, we'll want to display migration advice in the CNB. Or if we don't drop the feature, we'll need to implement support for it in the CNB.

GUS-W-17140864.