Make milli use edition 2021 (#4770)

* Make milli use edition 2021

* Add lifetime annotations to milli.

* Run cargo fmt

milli/src/documents/serde_impl.rs

@@ -56,7 +56,7 @@ impl<'a, 'de, W: Write> Visitor<'de> for &mut DocumentVisitor<'a, W> {
         Ok(Ok(()))
     }
 
-    fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn expecting(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "a documents, or a sequence of documents.")
     }
 }