Add Xcode 15 CI for Storage #5998

Job	Run time
changes	5s
spm-binary-cron	0s
check	3m 29s
cmake-prod-db (macos-12, (default))	32m 57s
cmake (macos-12)	31m 12s
sanitizers (macos-12, asan)	31m 14s
cmake-prod-db (macos-12, test-db)	33m 24s
cmake (ubuntu-latest)	19m 49s
xcodebuild (iOS)	44m 45s
sanitizers (macos-12, tsan)	29m 14s
xcodebuild (tvOS)	40m 24s
xcodebuild (macOS)	26m 48s
pod-lib-lint (FirebaseFirestoreInternal.podspec)	27m 22s
spm-binary	3m 38s
pod-lib-lint (FirebaseFirestore.podspec)	35m 46s
pod-lib-lint-cron	0s
pod-lib-lint (FirebaseFirestoreSwift.podspec)	27m 16s
spm-source	19m 51s
Check all required Firestore tests results	1s
	6h 47m 15s

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