oskgo has created PR #691: Remove `oldtime` feature from chrono dependency in mongo-rust-driver

Issue Text:
Remove `oldtime` feature from chrono dependency to eliminate a transitive dependency on a deprecated version of the time crate which has [a vulnerability](https://rustsec.org/advisories/RUSTSEC-2020-0071).

The issue was found using [cargo-audit](https://github.com/RustSec/rustsec/tree/main/cargo-audit), which you might want to add to CI.