The Andrew Lloyd Webber and T.S. Eliot musical “Cats,” which has earned $1.43 billion around the world and was long the butt of David Letterman jokes (until the Broadway version of the show closed last September), will celebrate its 20th birthday as London’s longest-running musical on Thursday. Eight million people have seen the spectacle in the British capitol, and the musical, hardly a favorite of the critics, has played in 300 cities from Hamburg to Tokyo.