Add SlackMessage::info (#23711)

I know it defaults to `info` but it's useful to be explicit about it in some cases.

src/Illuminate/Notifications/Messages/SlackMessage.php

@@ -83,6 +83,18 @@ class SlackMessage
      */
     public $http = [];
 
+    /**
+     * Indicate that the notification gives information about an operation.
+     *
+     * @return $this
+     */
+    public function info()
+    {
+        $this->level = 'info';
+
+        return $this;
+    }
+
     /**
      * Indicate that the notification gives information about a successful operation.
      *