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.