Suffering its worst ratings in 20 years, CNN is going Hollywood.In the past few weeks, the No. 3 cable news channel has started seeking out reality-show ideas and big-name stars not afraid to talk politics. They have even begun working on a late-night talk show, The Post has learned.Convinced that its current programming badly needs updating, CNN execs have been making the rounds of Hollywood’s top talent agencies — something entertainment networks do several times a year but a first for the old-line news channel.In a series of conference calls, the network has also been soliciting ideas from the same producers who supply reality shows to channels like Bravo, Discovery and History, according to sources.