media_update_7211() incorrectly calls image_style_flush() with a string

image_style_flush should be called with an array. (with at least an entry with key = 'name and the value as now is passed as string)

solution 1:

function media_update_7211() {
  image_style_flush(array('name' => 'square_thumbnail'));
  ...
}

Solution 2: to rename an image style you can simply:

$style = image_style_load('square_thumbnail');
if ($style) {
 $style['name'] = 'media_thumbnail';
  image_style_save($style);

image_style_save() will figure out it is a rename and will call image_style_flush().

I tested solution 2 locally and it works.