Downgrade swift-tools-version to 6.0 for broader compatibility

[JonyFang]

Summary

This PR makes QuickLayout accessible to developers using stable Xcode versions by downgrading the Swift tools version requirement.

Changes

• Downgrade swift-tools-version from 6.2 to 6.0
• Update swift-syntax dependency from 602.0.0 to 600.0.0
• Fix Demo project iOS deployment target from 26.0 to 15.0

Rationale

• Swift 6.2 requires Xcode 26, which is the latest version. Swift 6.0 (Xcode 16.0+) has broader adoption and allows more developers to use this library.
• swift-syntax version must match the Swift tools version (600.x for Swift 6.0).
• Demo iOS deployment target was 26.0, which is inconsistent with Package.swift's platforms: [.iOS(.v15)] declaration. Changed to 15.0 for consistency.

Verification

• swift package resolve succeeds
• xcodebuild -scheme QuickLayout -destination 'platform=iOS Simulator,name=iPhone 16' build succeeds
• Demo project builds successfully

[meta-cla]

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[constantine-fry]

@JonyFang, thank you for working on it. Happy to merge it once you sign the CLA. Let me know if you concerns.

[JonyFang]

@JonyFang, thank you for working on it. Happy to merge it once you sign the CLA. Let me know if you concerns.

Thanks for the quick response! I've already signed the CLA. The confirmation should appear shortly.

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[meta-codesync]

@constantine-fry has imported this pull request. If you are a Meta employee, you can view this in D88010997.

[meta-codesync]

@constantine-fry merged this pull request in f723e5a.