Update yii2-bootstrap dependency to Bootstrap 4

[raimon-segura]

Hi!,

It would be nice to upgrade the "yiisoft/yii2-bootstrap": "*", dependency from composer.json to Bootstrap 4 .. maybe using another repository name?

I've checked the sources and I found one usage of Bootstrap3 in Alert.php

• "use yii\bootstrap\Widget;" to extend the class, with ""use yii\bootstrap4\Widget;" should work.
• Also, it uses the css classes "success", "warning", etc... but its only a css class string.

Meanwhile, as a workaround, I'm using a "copy" of Alert.php class

Thanks!