[Explorer] Fix chrono deprecation warning for Utc.timestamp
Summary:
Utc.timestamp has been deprecated in a recent version of the chrono crate.
Switching to Utc.timestamp_nanos fixes this.
Test Plan:
- Build the explorer, no warnings
- Timestamps for blocks on the explorer are still correct
Reviewers: Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D12981