Use Swift 6.2

[rgoldberg]

Increase minimum supported macOS building version from 14 Sonoma to 15 Sequoia:

• Remove Xcode dependency in favor of swift?
• Swift cleanup:
  • Remove non-closure arguments for withTaskGroup
  • Task.detached ➡ Task
• Swift 6.2.3
  • Deploys to macOS 11+
  • SwiftSetting
    • .defaultIsolation(_:_:)
    • .enableUpcomingFeature("NonisolatedNonsendingByDefault")
    • .strictMemorySafety() & unsafe
    • .treatAllWarnings(as: .error)
  • Swift Testing 6.2.3
    • exit tests
      • stdout & stderr capturing?
• Swift 6.1
  • Deploys to macOS 10.13+
• SwiftLint
  • Remove some unused_import: allowed_transitive_imports: from .swiftlint.yml

Vestigial:

• Remember to update Xcode dependency to 26 in Homebrew formulae.
• Built-in shell commands:
  • /usr/bin/jq
  • /usr/bin/trash