Change connection-closed log message to debug

Previously it was info, and it's the only connection pool-related
log message at that high a level.

JAVA-4189

Author: jyemin

driver-core/src/main/com/mongodb/internal/connection/DefaultConnectionPool.java

@@ -535,8 +535,8 @@ public UsageTrackingInternalConnection create(final boolean initialize) {
         @Override
         public void close(final UsageTrackingInternalConnection connection) {
             connectionPoolListener.connectionRemoved(new ConnectionRemovedEvent(getId(connection), getReasonForClosing(connection)));
-            if (LOGGER.isInfoEnabled()) {
-                LOGGER.info(format("Closed connection [%s] to %s because %s.", getId(connection), serverId.getAddress(),
+            if (LOGGER.isDebugEnabled()) {
+                LOGGER.debug(format("Closed connection [%s] to %s because %s.", getId(connection), serverId.getAddress(),
                                   getReasonStringForClosing(connection)));
             }
             connection.close();