chrono: Address deprecation warnings

[Turbo87]

warning: use of deprecated associated function `chrono::DateTime::<Tz>::from_utc`: Use TimeZone::from_utc_datetime() or DateTime::from_naive_utc_and_offset instead
   --> src/posts.rs:161:38
    |
161 |     chrono::DateTime::<chrono::Utc>::from_utc(
    |                                      ^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default


warning: use of deprecated associated function `chrono::NaiveDate::from_ymd`: use `from_ymd_opt()` instead
   --> src/posts.rs:162:28
    |
162 |         chrono::NaiveDate::from_ymd(year, month, day).and_hms(0, 0, seconds),
    |                            ^^^^^^^^


warning: use of deprecated method `chrono::NaiveDate::and_hms`: use `and_hms_opt()` instead
   --> src/posts.rs:162:55
    |
162 |         chrono::NaiveDate::from_ymd(year, month, day).and_hms(0, 0, seconds),
    |                                                       ^^^^^^^