go feature: support pseudo-versions "stable" and "oldstable"

The GH action https://github.com/actions/setup-go has the notion of "stable" and "oldstable". This nicely supports the way the Go toolchain is supported always in versions 1.N and 1.N-1.

To my understanding, this "stable" is equivalent to "latest" in the go dev container feature. To easily test with stable and oldstable toolchain versions, I would like to propose to support also the pseudo-version "oldstable" to parallel the GH go-setup action mechanics. In this view, I propose to support also "stable" as an alias to "latest".