Use swiftlang SwiftSyntax repository

[afollestad]

Summary

• switch SwiftSyntax dependency URL from apple/swift-syntax.git to swiftlang/swift-syntax
• update tracked Package.resolved files to match the canonical repository URL

Why?

SwiftSyntax moved from the apple GitHub organization to the swiftlang organization. Downstream package graphs can now include both URLs when one dependency still references apple/swift-syntax.git and another references swiftlang/swift-syntax. SwiftPM normalizes both to the same package identity, swift-syntax, which produces duplicate identity warnings and extra repository update work during package resolution.

Using the canonical swiftlang/swift-syntax URL keeps Knit aligned with the current upstream repository location and avoids contributing the legacy URL to downstream dependency graphs.

Validation

• swift package resolve
• swift build -v
• swift test

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[gilgtm]

🤖 Approved after deep-dual review; no findings.

[afollestad]

CC @luispadron @efirestone

[afollestad]

Also, it looks like Rulesets in this repo might need to be updated to match the correct versions for build-knit-swift?