Platform versions of Swift Configuration

[czechboy0]

Right now they're pretty high (macOS 15-aligned), which can prevent adoption in many libraries. Let's see how easily we can at least drop them to the more common macOS 10.15-aligned ones.

[adam-fowler]

grpc v2 and valkey-swift have no platform requirements in the package.swift (or at least limited). They just add @available markup everywhere. This means other packages can include swift-configuration and still support older OS'es but the swift-configuration features are only available for the platforms it supports.

[czechboy0]

I see, so the ask here isn't really to make this library work on anything pre-macOS 15.0 aligned, just to be able to depend on it and also offer macOS 15.0-aligned APIs downstream?

[adam-fowler]

I see, so the ask here isn't really to make this library work on anything pre-macOS 15.0 aligned, just to be able to depend on it and also offer macOS 15.0-aligned APIs downstream?

Pretty much. Obviously if you can remove the 15.0 requirement that'd be even better

[czechboy0]

Obviously if you can remove the 15.0 requirement that'd be even better

I tried that for a bit, unfortunately that started growing into a pretty substantial, and backwards-looking refactor that I'd like to avoid for now.

However, I did make the change to use availability annotations: #48