Revert "build: increase MSRV to 1.81.0"

This reverts commit ffe6efe819.

src/contact.rs

@@ -805,6 +805,7 @@ 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")