Revert "build: increase MSRV to 1.81.0"

This reverts commit ffe6efe819.

src/imap.rs

@@ -1301,7 +1301,7 @@ impl Session {
     /// Returns the last UID fetched successfully and the info about each downloaded message.
     /// If the message is incorrect or there is a failure to write a message to the database,
     /// it is skipped and the error is logged.
-    #[expect(clippy::too_many_arguments)]
+    #[allow(clippy::too_many_arguments)]
     pub(crate) async fn fetch_many_msgs(
         &mut self,
         context: &Context,
@@ -2689,6 +2689,7 @@ mod tests {
         }
     }
 
+    #[allow(clippy::too_many_arguments)]
     async fn check_target_folder_combination(
         folder: &str,
         mvbox_move: bool,