Jettison Python 3.7 as it won't work with Django 4

[dotysan]

• add build target for Django 4.2LTS
• also update 2.2 & 3.2 LTS revs
• upate actions to use Node.js 16

[subzero10]

Merging this means that we officially stop support for python 3.7.

I'm wondering if we should wait until Python 3.7 is officially "End of Life" before we merge it 🤔 .

[dotysan]

So yeah, it's way above my pay grade to make that decision to remove support for Python 3.7.

I've hacked up a python.yaml that still builds Django 1/2/3 with Python 3.7 but skips it with Django 4.

Will create new PR and close this one.