WASHINGTON — The Supreme Court on Wednesday upheld a Biden administration regulation on the nearly impossible-to-trace weapons called ghost guns, clearing the way for continued serial numbers, background checks and age verification requirements to buy them in kits online.Seven justices joined the opinion, authored by Justice Neil Gorsuch, upholding the rule. Two justices, Clarence Thomas and Samuel Alito, dissented.(snip)Ghost guns are any privately made firearms without the serial numbers that allow police to trace weapons used in crime. The 2022 regulation was focused on kits sold online with everything needed to build a functioning firearm — sometimes in less than...