ci: Use `macos-15` VM image for Xcode 16.1 tests #1554

Job	Run time
Cocoapods Integration Tests - macOS	13s
changes	5s
Codegen Test Configurations - macOS	4m 12s
Run swift build for SPM packages (apollo-ios)	1m 6s
Run swift build for SPM packages (apollo-ios-codegen)	26s
Run swift build for SPM packages (apollo-ios-pagination)	30s
Run Tuist Generation	0s
Verify CLI Binary Archive - macOS	0s
Verify Frontend Bundle Latest - macOS	0s
${{ matrix.name }}	0s
	6m 32s

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