Fix clippy::derivable_impls warning

```
error: this `impl` can be derived
  --> futures-util/src/stream/select_with_strategy.rs:33:1
   |
33 | / impl Default for PollNext {
34 | |     fn default() -> Self {
35 | |         Self::Left
36 | |     }
37 | | }
   | |_^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#derivable_impls
   = note: `-D clippy::derivable-impls` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::derivable_impls)]`
help: replace the manual implementation with a derive attribute and mark the default variant
   |
 9 + #[derive(Default)]
10 | pub enum PollNext {
11 |     /// Poll the first stream.
12 ~     #[default]
13 ~     Left,
   |
```

Author: taiki-e

futures-util/src/stream/select_with_strategy.rs

@@ -5,9 +5,10 @@ use futures_core::task::{Context, Poll};
 use pin_project_lite::pin_project;
 
 /// Type to tell [`SelectWithStrategy`] which stream to poll next.
-#[derive(Debug, PartialEq, Eq, Copy, Clone, Hash)]
+#[derive(Debug, Default, PartialEq, Eq, Copy, Clone, Hash)]
 pub enum PollNext {
     /// Poll the first stream.
+    #[default]
     Left,
     /// Poll the second stream.
     Right,
@@ -30,12 +31,6 @@ impl PollNext {
     }
 }
 
-impl Default for PollNext {
-    fn default() -> Self {
-        Self::Left
-    }
-}
-
 enum InternalState {
     Start,
     LeftFinished,