Update "Supports PHP:" value for all themes

[creativecoder]

We've been adding declare( strict_types=1 ); to theme PHP files because of WP.com deploy requirements.

This mode was added in PHP 7. Currently most (all?) themes in this repo declare "Support PHP: 5.x" in the style.css header.

We need to update this to correctly require PHP 7.0+, to indicate that themes may trigger errors if used with older PHP versions.

[creativecoder]

I've restored [Type] Bug for this issue--fixing incorrect PHP compatibility information is not an enhancement.