ci: run xOS 27 tests on tahoe-27 runners

[github-actions]

Note

The pull request "ci: run xOS 27 tests on tahoe-27 runners" was created by @itaybre and did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Summary

• Move iOS/tvOS/visionOS/watchOS 27 unit tests and UI tests from xcode-beta to tahoe-27 runners
• Add macOS 27 and Catalyst 27 unit test jobs, now possible with macOS 27 runners
• Remove standalone xcode27-test.yml workflow and its run_xcode27_for_prs file filter (Xcode 27 builds are covered by the main test workflow)
• Drop allow_prerelease and continue_on_error from matrix entries and workflow references that no longer need them

#skip-changelog

[linear-code]

COCOA-1917