State current GHC support window (and other support windows)

[ffaf1]

What is wrong with the docs?
We miss a clear specification of what exactly we do mean with "support" for GHC. Compiling? Driving?
Let us clarify that in CONTRIBUTING.md.

Also we might or might not want to switch from “𝑥 years” to “last 𝑦 major versions”. No, we will do it in another ticket, once we agree to document what we are doing now.

Related PR: #11530