Deprecate and eventually sunset `runtime.txt` support

[edmorley]

As of #272, the buildpack now supports the .python-version file for configuring the Python version.

The .python-version file is superior in several ways:

• it is commonly supported/used by other tools (unlike runtime.txt, which is Heroku proprietary)
• it supports the major Python version syntax (eg 3.X rather than 3.X.Y), so supports automatic security updates without needing to manually bump the patch version each time.

As such, we should deprecate runtime.txt support in the CNB, and then eventually remove support from the CNB entirely.

For the classic buildpack, see:
heroku/heroku-buildpack-python#1642

GUS-W-16878239.
GUS-W-16878263.

[edmorley]

Deprecation announcement:
https://devcenter.heroku.com/changelog-items/3141