chore(build): update setup-ruby github action for macos-15 runners

[thisisabhash]

Issue #

Github actions are failing for setup-ruby after updating runners to macos-latest which correspond to macos-15 now.
https://github.com/aws-amplify/amplify-swift/actions/runs/17246673926

Description

Updating setup-ruby to v1.256.0.

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.