Use Swift 6 when available & upgrade to Xcode 16.2

[rebello95]

After some discussion in apple/swift-protobuf#1729, I noticed that the compilation error when building with Swift 6 is actually an issue with the config in Tests/UnitTests/ConnectLibraryTests/buf.gen.yaml which was still running on 1.25.2 instead of 1.28.2 like other parts of the project.

Changes in this PR:

• Updates that directory to run on 1.28.2 which includes apple/swift-protobuf@3bc7630 and resolves the problem described in the aforementioned issue
• Switches CI to use Xcode 16.2 and macOS 15
• Switches our Package.swift to use Swift 6 when available via swiftLanguageVersions: [.version("6"), .v5]
• Updates the Eliza SPM app to use Swift 6

This is related to #310, but we will not be able to actually implement those until we fully switch to using Swift 6 (a breaking change).

[jhump]

Nice!

[eseay]

Let me know what you think here. As far as buf.gen.yaml and ci.yaml - that all looks great.

[eseay]

I think what we may want to do instead is keep the swift-tools-version set at 5.6 and then specify swiftLanguageVersions: [.version("6"), .v5] (instead of swiftLanguageModes which is new and Swift 6/Xcode 16 only).

What this will do is allow us to compile with Swift 6 when using the package in Xcode 16, but it will continue to let the package compile with Xcode 15.4 as well. (docs here). I think that's what we want, because if we increase the required swift-tools-version, I believe that would technically be considered a breaking change, since any dependent packages that are also set to 5.6 would need to increase their tools version to at least the value specified by Connect-Swift.

I could also be wrong here, but just wanted to share that since I've also been going through this exercise of updating some of my packages to compile with Swift 6 while still wanting them to be usable on Xcode 15.4.

Thoughts?

[rebello95]

Thanks for this writeup @eseay.

I think you're correct based on the docs you linked, particularly the section on:

If instead you would just like to use Swift 6 language mode when it’s available (while still continuing to support older modes) you can keep a single Package.swift and specify the version in a compatible manner

I pushed an update with your recommendation 🙏🏽

[eseay]

Awesome!