Rename "diagonal striped background"

BUR #34415 has been approved by @nonamethanks.

rename diagonal_striped_background -> diagonal-striped_background

For consistency with other diagonal-striped tags.

The reason it's named without the hyphen is because that tag was deprecated following topic #25660, ideally both that one and vertical-striped background should be undeprecated following the killing of diagonal stripes and vertical stripes.

The bulk update request #34415 (forum #322082) has been approved by @nonamethanks.