Remove Decoder Injector interface from webhook

[RealAnna]

Unfortunately, controller runtime from version 0.15.0 onward does not support the injection of decoders anymore. So to prevent future failures we do need to change our lifecycle operator main to explicitly initialize the decoders. This has been already tested here: https://github.com/keptn/lifecycle-toolkit/blob/poc/scheduling_gates/operator/main.go#L351

[geoffrey1330]

Hi @thisthat I want to contribute to this. could you please assign it to me.