[rust] Fix Clippy warnings about DateTime

Summary:
Received a warn that the chrono::NaiveDateTime::from_timestamp is deprecated. Replaced this with recommended method which also simplified our code

Depends on D12176

Test Plan:

• cargo check
• CI - there are some unit and integration tests that rely on timestamps

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12178