[3.14] gh-140189: Use macos-14 runner for iOS CI tests (GH-142444) (#142494)

Switch to the macos-14 runner for iOS CI, working around some instability
in the macos-15 runner.
(cherry picked from commit 91884838bc3c47e02ab6099e6f9b12d80a0abae2)

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>