Todd Spangler NY Digital EditorAt this year’s Cannes Film Festival, heads will turn at a new May-December romance: TikTok, which is 6 years old, has hooked up with the venerable fest, now celebrating its 75th anniversary.How is it that Cannes — which, in years past, notoriously banned social-media posting from its famous red-carpeted steps — is now welcoming TikTok as one of its elite sponsors? For the festival, the partnership is aimed at pulling Cannes forward into the 21st century, lending it a sheen of techno-hipness and ideally attracting younger film fans. For TikTok, which boasts more than 1 billion monthly users worldwide, pairing with the Cannes Film Festival is an effort to show filmmakers that the app is more than just a place for teenagers to lip-sync to the latest viral dance craze.