Drop python 3.8 support.

[bckohan]

Python 3.8 security support just ended with the release of 3.13 on 10/7/2024. Normally if there wasn't a compelling reason to drop support I wouldnt so soon, but dropping 3.8 will do two things:

• Eliminate all of the imports of Annotated from typing_extensions as Annotated was added to stdlib in 3.9
• Fix the cls parameter overload issue. Typer unwisely used cls as the argument name when overriding the click.Group class to use for commands. This causes issues with how 3.8 passes arguments in meta classes and type constructors, leading to weirdness. This may also make Overriding the command group class does not work. #128 easier to fix.