Reapply "build: increase MSRV to 1.81.0"

This reverts commit 9d331483e9.

src/contact.rs

@@ -802,7 +802,6 @@ impl Contact {
         }
 
         let mut name = sanitize_name(name);
-        #[allow(clippy::collapsible_if)]
         if origin <= Origin::OutgoingTo {
             // The user may accidentally have written to a "noreply" address with another MUA:
             if addr.contains("noreply")