time: import Future trait from std instead of futures_core (#6884)

tokio-util/src/time/mod.rs

@@ -8,7 +8,7 @@
 //!
 //! This type must be used from within the context of the `Runtime`.
 
-use futures_core::Future;
+use std::future::Future;
 use std::time::Duration;
 use tokio::time::Timeout;