Unpin addressable in customerio.gemspec

[olleolleolle]

This PR unpins the specific dependency addressable, to allow 2.9+.

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Note

Low Risk
Low risk dependency change limited to the gemspec; main risk is unexpected behavior changes if consumers resolve a newer addressable version.

Overview
Relaxes the addressable runtime dependency constraint in customerio.gemspec from ~> 2.8.0 to ~> 2.8, allowing bundlers to pick up newer compatible addressable releases (e.g., 2.9.x) without changing any library code.

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