Former Mayor Rudy Giuliani has decided not to run for governor next year - but will run for U.S. Senate instead, sources told the Daily News. A source familiar with Giuliani's thinking said the failed presidential candidate has been telling people he plans to run against Democratic Sen. Kirsten Gillibrand in 2010 to fill out the remaining two years of Hillary Clinton's term. If elected, the source said, he could use that as a stepping stone to run for President in 2012 - rather than run for re-election to the Senate. A Giuliani spokeswoman downplayed the reports. "Rudy has a...