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.

Note: For the classic buildpack, we'll be also adding .python-version support there soon (which I'd like to do before deprecating runtime.txt in the CNB). However, for the classic buildpack we'll never want to fully remove runtime.txt support - and instead only deprecate it - to at least encourage most users to migrate over before they eventually migrate to the CNB (xref heroku/heroku-buildpack-python#1642).

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