[CocoaPods] Use new Trunk API endpoint to get podspecs

See https://github.com/CocoaPods/search.cocoapods.org/commit/d6ac677b00a3b5b649f64ad6e2d3f30721545bd8#commitcomment-8340174 and https://github.com/CocoaPods/trunk.cocoapods.org/pull/109.

Shields is the last remaining (known) consumer of that search API, which we're looking to remove in order to allow for backend architecture enhancements.