Replace chrono with jiff (#8008)

`jiff` is more modern, and seem to be where the ecosystem is heading.

crates/egui_demo_app/src/lib.rs

@@ -9,9 +9,10 @@ pub use wrap_app::{Anchor, WrapApp};
 
 /// Time of day as seconds since midnight. Used for clock in demo app.
 pub(crate) fn seconds_since_midnight() -> f64 {
-    use chrono::Timelike as _;
-    let time = chrono::Local::now().time();
-    time.num_seconds_from_midnight() as f64 + 1e-9 * (time.nanosecond() as f64)
+    jiff::Zoned::now()
+        .time()
+        .duration_since(jiff::civil::Time::midnight())
+        .as_secs_f64()
 }
 
 /// Trait that wraps different parts of the demo app.