Drop support for Python 3.8

[StevenMaude]

We test with Python 3.8, and also produce the build with Python 3.8. Python 3.8 is end-of-life as of 2024-10-07.

Existing users with local setups may be using an existing Python 3.8 installation to run this software. How do we get those users migrated? The solution there may be related to #277.

Are there other places where the Python version used for OpenSAFELY CLI interact with other components, and we can't upgrade from Python 3.8? (I think this should be entirely independent, but I'll prompt the question in case not.)