Drop old "porting" documentation as it fails htmlvalidation

[akurtakov]

A lot of "porting" documents fail htmlvalidation (#3238) and it's shipped all the way back to 4.5 (from 2015!).
In order to finish the htmlvalidation task my proposal is to drop the porting documentation that's older than 8 releases (same period as in https://github.com/eclipse-platform/eclipse.platform/blob/master/docs/ApiRemovalProcess.md ) in order to make the task feasible (and actually ship less most likely useless nowadays content).

[ptziegler]

2 years sounds reasonable to me. Otherwise the number of documents will just increase indefinitely.

[HannesWell]

Sounds good to me as well.
If possible, you could add a link to older porting guides at the end.