template_preprocess_file_link() now retains the link variable as a render array

Prior to 8.8.x template_preprocess_file_link() turns the link variable into a string.

This prevents other modules/themes from modify the link attributes etc.

From 8.8.0-alpha1, this is retained as a render array, so it can be modified

Before

$variables['link'] = Link::fromTextAndUrl($link_text, Url::fromUri($url, $options))->toString();

After

$variables['link'] = Link::fromTextAndUrl($link_text, Url::fromUri($url, $options))->toRenderable();