CocoaPods/Specs — pull request #6067 by fkazak (2013-12-11 14:29:30+00:00)
Title: Decreased GoogleProtobuf .podspec deployment target from 7.0 to 5.0

protobuf itself doesn't really depend on iOS SDK version at all.
This particular podspec was tested locally on both simulator and devices with iOS 5 and 6 and caused no problems.
I assume it was set to 7.0 just to ensure Xcode 5 usage.