Media: Use max-width for default captions.

This alters the HTML output of the image caption shortcode to use
`max-width` instead of `width` to improve compatibility with
flexible layouts.

Props aaronrutley, desrosj.
Fixes #33981.


git-svn-id: https://develop.svn.wordpress.org/trunk@41724 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/shortcode.php

@@ -477,7 +477,7 @@ EOF;
 			),
 			array(
 				'[caption caption="test" width="2"]<div>hello</div>[/caption]',
-				'<div style="width: 12px" class="wp-caption alignnone"><div>hello</div><p class="wp-caption-text">test</p></div>',
+				'<div style="max-width: 12px" class="wp-caption alignnone"><div>hello</div><p class="wp-caption-text">test</p></div>',
 			),
 			array(
 				'<div [gallery]>',