Replace directories-next dependency with directories (#4661)

`directories-next` was created because `directories` was not maintained
at the time. However, `directories` has gotten active maintainership
since, and it has received more updates than `directories-next`.

`directories` also has more recent downloads than its `next`
counterpart, so it might make sense to switch to it, to avoid
unnecessary duplicate dependencies, where a project depends transitively
on both `directories` and `directories-next`.

The main question is whether we depend on any specific behavior from
`directories-next`.

crates/eframe/src/epi.rs

@@ -367,7 +367,7 @@ pub struct NativeOptions {
     pub persist_window: bool,
 
     /// The folder where `eframe` will store the app state. If not set, eframe will get the paths
-    /// from [directories_next].
+    /// from [directories].
     pub persistence_path: Option<std::path::PathBuf>,
 }