Bump the zip crate from 3 to 9 in the workspace Cargo.toml and
adapt to the API change in 2.0+: last_modified_time now takes
Option<DateTime> instead of DateTime directly.
- crates/io/src/lib.rs: wrap DateTime::default() in Some(...)
- crates/geometry/src/tagged_mesh.rs: same fix in the 3MF writer
Closes#134