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.

Review the removal already started in issue #2700, then trace which transitive dependencies still set or use Python 3.8 or 3.9. The work is done when both versions are removed without breaking those dependencies, including bumping protobuf or similar dependencies if required.

Written by the indexing model from the issue text.