build: clippy disallow futures::pin_mut macro (#7016)

`std` has had `pin!` macro for some time, there is no need for us to use
the older alternatives. Cannot disallow `tokio::pin` because tokio
macros use that.

control_plane/src/pageserver.rs

@@ -605,7 +605,7 @@ impl PageServerNode {
                 eprintln!("connection error: {}", e);
             }
         });
-        tokio::pin!(client);
+        let client = std::pin::pin!(client);
 
         // Init base reader
         let (start_lsn, base_tarfile_path) = base;