deprecate ForeverHandle.MarkSupervised()

[ronnno]

This method became obsolete after recent changes to the supervision mechanism.

We use it outside of govnr only in one case where we want to bypass the supervision mechanism. near it, this comment appears: //TODO use real supervision?

In this case, we should either conform to the supervision semantics or stop using govnr altogether, or otherwise replace this method with a clearer ForeverNoShutdownSupervision() (which will not return a ForeverHandler but only a channel which is closed upon termination instead), or rename this workaround method so it's clear that it does the opposite of what it current name suggests: either MarkUnsupervised() to RunForeverWithoutSupervision()

Once the method is deprecated it should be marked as in internal function to govnr and not exposed to consuming modules

[electricmonk]

I'd like to wait with any further changes to govnr until we decide what we want to do with it