Remove cargo audit ignores once chrono advisory is resolved

[tarikeshaq]

With #4589, we added cargo audit ignores for two security advisories. This ticket is to remove the ignores

This means one of two things:

• Either remove chrono all together and use time directly
• If chrono resolves the issue and releases a fixed version, upgrade to that

We should do this once enough time has passed and/or:

• Bump time to 0.3 chronotope/chrono#578 is merged
• CVE-2020-26235 advisory for time 0.1 dependency chronotope/chrono#602 is fixed

┆Issue is synchronized with this Jira Task
┆Epic: Important backlog

[jhpratt]

Note that chrono itself has the same issue, not just transitively via time. Chrono updating to time 0.3 by itself is not sufficient.

[tarikeshaq]

Thank you! Looks like they have an issue for that: chronotope/chrono#499, adding it here for future reference.