refactor(swarm): update ConnectionDenied::new

This allows us to input `String`'s and `&str` on ConnectionDenied::new() as there's [`From<String> for Box<dyn Error + Send + Sync>`](https://doc.rust-lang.org/std/error/trait.Error.html#impl-From%3CString%3E-for-Box%3Cdyn+Error+%2B+Send+%2B+Sync,+Global%3E).

Pull-Request: #4530.

Author: jxs

swarm/src/lib.rs

@@ -1728,9 +1728,9 @@ pub struct ConnectionDenied {
 }
 
 impl ConnectionDenied {
-    pub fn new(cause: impl error::Error + Send + Sync + 'static) -> Self {
+    pub fn new(cause: impl Into<Box<dyn error::Error + Send + Sync + 'static>>) -> Self {
         Self {
-            inner: Box::new(cause),
+            inner: cause.into(),
         }
     }