Fix cargo clippy issues

geneit_backend/src/services/mail_service.rs

@@ -27,7 +27,7 @@ pub async fn send_mail<D: Display>(to: &str, subject: &str, body: D) -> anyhow::
     let mailer = mailer.build();
 
     mailer.send(&email)?;
-    log::debug!("A mail was sent to {} (subject = {})", to, subject);
+    log::debug!("A mail was sent to {to} (subject = {subject})");
 
     Ok(())
 }