Support Swift6 version to Birthday sample app. #823

Job	Run time
spm-build-test (macos-15, macosx)	1m 44s
spm-build-test (macos-15, iphonesimulator)	3m 34s
pod-lib-lint (macos-13, GoogleSignIn.podspec)	11m 16s
pod-lib-lint (macos-13, GoogleSignIn.podspec, --use-static-frameworks)	12m 18s
pod-lib-lint (macos-13, GoogleSignInSwiftSupport.podspec, --use-static-frameworks)	14m 42s
pod-lib-lint (macos-13, GoogleSignInSwiftSupport.podspec)	11m 28s
	55m 2s

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