Remove Python 3.8 and 3.9

I have started the removal in #2700, but noticed that a proper removal may require more thinking about how to do this without breaking transitive deps still setting/using 3.8. Probably this ticket will involve bumping protobuf or similar deps to avoid transitive deps using 3.8.

EDIT: Since from 2025 October 3.9 is EOL, we should remove 3.9 as well.

检查 issue #2700 中已经开始的移除工作，然后追踪哪些传递依赖仍在设置或使用 Python 3.8 或 3.9。当两个版本都被移除且不破坏这些依赖（包括在需要时升级 protobuf 或类似依赖）时，工作就完成了。

由索引模型根据 Issue 内容生成。