Upgrade officially supported Python version to 3.8 or later

[yatbear]

We are stuck at 20221213 tf-nightly, since TensorFlow stopped supporting Python3.7 (20221214 and later nightlies do not have cp37 whls). This is blocking #6128.

Python 3.7 also reached EOF: https://devguide.python.org/versions/#supported-versions.