Downgrade non-IO connection closed log from info to debug (#4441)

When unmeshed clients (such as cloud provider health probes) connect to
the inbound proxy, the connection is refused because it lacks mTLS. This
produces a 'Connection closed' log at INFO level with the error
'direct connections must be mutually authenticated'. In environments
like AKS/EKS multicluster gateways, load balancer TCP health probes
trigger this message at high frequency, flooding logs and obscuring
actionable errors.

Downgrade this catch-all error log from info! to debug! so that these
expected, non-actionable connection closures no longer spam production
logs while remaining available when debug-level logging is enabled.

Fixes linkerd/linkerd2#10203

Signed-off-by: rohansood10 <rohansood10@users.noreply.github.com>
Co-authored-by: rohansood10 <rohansood10@users.noreply.github.com>

Author: rohansood10

linkerd/app/core/src/serve.rs

@@ -7,7 +7,7 @@ use futures::prelude::*;
 use linkerd_error::Error;
 use linkerd_proxy_transport::AddrPair;
 use tower::util::ServiceExt;
-use tracing::{debug, debug_span, info, instrument::Instrument, warn};
+use tracing::{debug, debug_span, instrument::Instrument, warn};
 
 /// Spawns a task that binds an `L`-typed listener with an `A`-typed connection-accepting service.
 ///
@@ -65,8 +65,8 @@ pub async fn serve<M, S, I, A>(
                                             );
                                         }
                                         Err(error) => {
-                                            info!(
-                                                error,
+                                            debug!(
+                                                %error,
                                                 client.addr = %client_addr,
                                                 server.addr = %server_addr,
                                                 "Connection closed"