Set chrono minimum version to v0.4.21 (#1272)
Set chrono minimum version to v0.4.21 (#1272)

* Set chrono minimum version to v0.4.20

Per [RUSTSEC-2020-0159](https://rustsec.org/advisories/RUSTSEC-2020-0159.html), `chrono` may segfault under certain circumstances. This bumps `chrono`'s version to `0.4.20` to mitigate the issue.

* Change chrono version to v0.4.21

Co-authored-by: Andrew Burkett <e6fc878e51f5cf6bfc37a81520712fde9f94611e@crowdstrike.com>
mozilla/sccache,mozilla/sccache,mozilla/sccache