xurl: replace deprecated postflight hook

[karbo-hub]

Replaces the deprecated postflight hook with the supported postflight_steps API, scoped to macOS. This removes Homebrew’s deprecation warning while retaining the quarantine-attribute cleanup.

[CLAassistant]

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[zhoujians]

Verified locally against current Homebrew on macOS (arm64): applying this diff clears the Calling postflightis deprecated! Usepostflight_steps instead. warning on brew info --cask xurl, and the new run + {{staged_path}} syntax is fully supported by the current install-steps DSL. Quarantine-stripping behavior is preserved. LGTM.

[ikunali]

@karbo-hub Could you please sign the CLA, so we can get rid of these annoying warnings ASAP?