Drop support for Python 3.11

[tiran]

Fromager currently supports Python >= 3.11. I propose to drop support for Python 3.11 and make Python 3.12 the minimum supported Python version. It would allow us to use Python 3.12 features and reduce our test matrix.

In upstream, Python 3.11 is in security-fix only mode. In downstream, we have stopped using Python 3.11 a while ago. Python 3.11 in RHEL 9 is reaching end of support in a few months.

https://docs.python.org/3/whatsnew/3.12.html#improved-modules

[tiran]

Download stats for Python 3.11 vs. 3.12 from https://pypistats.org/packages/fromager