fix: Upgrade the Dockerfile to python 3.12 and sever from devstack

edx-enterprise now depends on py 3.11 or 3.12 to run unit tests.

Also, permanently SEVER the test container from devstack since we are
transitioning these in-code docker environments to no longer be part of
devstack.  If we need a devstack app for edx-enterprise, we need to add
it to the devstack repo now.