Rather than pinning every macOS job to macOS 15, only run the older CodeQL CLI
versions (which need Xcode 16, and so macOS 15) there. Newer versions, which
support Swift 6.2, run on the latest macOS runner with its default Xcode, so we
keep exercising the common latest-CLI-on-latest-runner combination.
To express this, `sync.ts` now supports an optional per-OS-entry
`codeql-versions` filter, used by `multi-language-autodetect` to send old
versions to `macos-15-xlarge` and new versions to `macos-latest-xlarge`.
`swift-custom-build` only runs `linked`/`default`/`nightly`, which all support
Swift 6.2, so it no longer needs the Xcode 16 step or a macOS 15 pin.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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