Bump Rust to 1.91.1 (#42561)

Release Notes:

- N/A

---------

Co-authored-by: Julia Ryan <juliaryan3.14@gmail.com>

crates/workspace/src/pane.rs

@@ -422,8 +422,9 @@ struct NavHistoryState {
     next_timestamp: Arc<AtomicUsize>,
 }
 
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Default, Copy, Clone)]
 pub enum NavigationMode {
+    #[default]
     Normal,
     GoingBack,
     GoingForward,
@@ -432,12 +433,6 @@ pub enum NavigationMode {
     Disabled,
 }
 
-impl Default for NavigationMode {
-    fn default() -> Self {
-        Self::Normal
-    }
-}
-
 pub struct NavigationEntry {
     pub item: Arc<dyn WeakItemHandle>,
     pub data: Option<Box<dyn Any + Send>>,