Update documentation to inform that heroku-24 buildstep platform does not include python2

[ianheggie-oaf]

Description

heroku-24 buildstep platform does not include python v2 - we should tell people that in the documentation

Also at the moment scraperwiki doesn't support python3 - we should fix or note that:

• scraperwiki-python should be upgraded to python3 on a new branch #1450

To Reproduce
Steps to reproduce the behavior:

1. try and run a python2 sript on morph.io with platform-24 once it is supported
2. See error