Downgrade to chrono 0.4.20

[mhammond]

This is a workaround to bring app-services and mozilla-central into agreement re supported chrono versions. See bug 1951517 for the m-c side of this.

See https://docs.rs/chrono/latest/src/chrono/datetime/mod.rs.html#92-110 - it looks like chrono just decided the "old" name wasn't good enough.

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• Quality: This PR builds and tests run cleanly
  • Note:
    • For changes that need extra cross-platform testing, consider adding [ci full] to the PR title.
    • If this pull request includes a breaking change, consider cutting a new release after merging.
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry in CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
• Dependencies: This PR follows our dependency management guidelines
  • Any new dependencies are accompanied by a summary of the due diligence applied in selecting them.

Branch builds: add [firefox-android: branch-name] to the PR title.

[mhammond]

This doesn't work in practice in m-c due to mozilla/glean#3071 - so while I think this patch is fine, I'll keep it open until we can actually upgrade m-c to 0.4.20. See also https://bugzilla.mozilla.org/show_bug.cgi?id=1951517