Reapply "build: increase MSRV to 1.81.0"

This reverts commit 9d331483e9.

src/smtp.rs

@@ -103,7 +103,7 @@ impl Smtp {
     }
 
     /// Connect using the provided login params.
-    #[allow(clippy::too_many_arguments)]
+    #[expect(clippy::too_many_arguments)]
     pub async fn connect(
         &mut self,
         context: &Context,