Rely on Jenkins 1.548+ so we can use TransientActionFactory

(Actually updating baseline to the current LTS, for more realistic testing.)

Using the new `TransientActionFactory` simplifies the code by letting us have a single factory which can be used for jobs (not just `AbstractProject`s but external monitors etc. too), folders (without a specific dependency on that plugin), and more exotic item types (there are several examples in Jenkins Enterprise).