Clarifying PEP 11 around dropping support

[brettcannon]

https://discuss.python.org/t/im-stepping-down-as-sponsor-of-emscripten/41064/6 has Pyodide asking how to interpret https://peps.python.org/pep-0011/#unsupporting-platforms and the fact it calls for building to fail to "potential users of the platform a chance to step forward and offer maintenance" while we have no concept of community support w/o a sponsor.

A couple of options that I can think of:

1. Take out the line about giving the community a chance to find support
2. Change it to say it's a chance to find a core dev sponsor
3. We don't require a build error, but we do require a notice in What's New

[gvanrossum]

I personally don't think we need to force a build error. Sure, it's likely in many cases that unsupported platforms will start developing build errors, and that's fine -- but at the same time there's a difference between "not supported" and "actively sabotaged". Given that most unsupported platforms have only a very small user base who care about Python, I doubt that we'll be inundated with bug reports. If an occasional one comes through, closing it with a reference to PEP 11 seems simple enough.

Given that the intended process is that a core dev sponsor must be found before support can be (re)instated, it makes sense to add a reminder to this section of the PEP.