Update macOS runner to macos-15 to fix deprecation (#570) #896

Job	Run time
spm-build-test (macos-15, macosx)	1m 41s
pod-lib-lint (macos-15, GoogleSignIn.podspec, --use-static-frameworks)	8m 25s
spm-build-test (macos-15, iphonesimulator)	2m 18s
pod-lib-lint (macos-15, GoogleSignInSwiftSupport.podspec)	5m 40s
pod-lib-lint (macos-15, GoogleSignInSwiftSupport.podspec, --use-static-frameworks)	5m 48s
pod-lib-lint (macos-15, GoogleSignIn.podspec)	6m 32s
	30m 24s

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