Support platform versions back to macOS 10.15, iOS 13.0, tvOS 13.0, and watchOS 6.0

[czechboy0]

Let's see how much work it'd be to reduce the deployment targets, macOS 13->11, and so on.

This would be all of the repos - generator, runtime, and transports.

// Edit: Possibly we could go as low as .macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .watchOS(.v6).

[andrewse02]

Hi, I actually did this today! You may have already seen that. It was very simple. There was one instance I found of iOS 16+/macOS 13+ code being used in swift-openapi-runtime. Here's my commit. It was this right here in Converter+Client.swift:

for parameter in parameters {
    renderedString.replace(
        "{}",
        with: parameter.description,
        maxReplacements: 1
    )
}

It was simple enough to wrap this in a if #available. Would you like me to make a PR from my fork?

[simonjbeaumont]

I think we should be OK to relax the deployment version.

@andrewse02 thanks for looking into it and for offering to put a PR together—that'd be great. 🙌

[denil-ct]

I was thinking of reducing it to iOS 12 as one of our apps still had 12 as the deployment target, and it was easy to change it for the generator and runtime. But for the transport since there is no concurrency support, I've hit a roadblock :(
Till iOS 13, there aren't any major issues, so I think we can reduce it to even iOS 13.
I have opened a PR for url session transport here.

If I need to use it in a iOS 12 environment is it doable in some other way? Maybe with the new macros can we make it generate both the completion style and async/await styles for the client.send method?

[czechboy0]

@andrewse02 Since @denil-ct was able to reduce the URLSession transport's deployment target by 3 versions in apple/swift-openapi-urlsession#7, can you try to do the same for the runtime and generator as well? It's okay if transports are more restrictive, as they're usually more tied to specific platform features, but if it's less restrictive, then adopters won't be able to use it anyway (due to the runtime package requiring a higher deployment target).

So let's try to go for the following: .macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .watchOS(.v6).

@denil-ct If you'd like, you could try a similar PR in https://github.com/swift-server/swift-openapi-async-http-client

[denil-ct]

Sure @czechboy0, will check it out.

[andrewse02]

Sure @czechboy0, I'll get on that!