Create multiple versions of a task instead of having backwards-compatibility code in it

[christian-kreuzberger-dtx]

Right now the tasks in this repo have backwards-compatible code in them.

It's better to have multiple versions of a task, e.g.:

https://github.com/microsoft/azure-pipelines-extensions/tree/master/Extensions/ServiceNow/Src/Tasks/UpdateChangeRequest

Note: We only aim to support the last 2-3 Keptn versions (e.g., 0.11.x, 0.10.x and 0.9.x).

Definition of Done

• Each task has a sub-folder for the current version (e.g., ${TaskName}v2, ${TaskName}v3) with the respective backwards compatibility
• Documentation added which task + version is compatible with which Keptn version