ci: Update Xcode to 15.4 #640

Job	Run time
spm-test	3m 14s
windows	2m 28s
linux	2m 17s
test (platform=iOS\ Simulator,OS=17.5,name=iPhone\ 15\ Pro\ Max, test)	3m 58s
test (platform=iOS\ Simulator,OS=17.5,name=iPhone\ 15\ Pro\ Max, build)	1m 35s
test (platform\=tvOS\ Simulator,OS=17.5,name\=Apple\ TV, test)	3m 27s
test (platform\=tvOS\ Simulator,OS=17.5,name\=Apple\ TV, build)	1m 24s
test (platform=watchOS\ Simulator,name=Apple\ Watch\ Series\ 9\ \(41mm\), build)	1s
test (platform=visionOS\ Simulator,OS=1.2,name=Apple\ Vision\ Pro, test)	1m 35s
test (platform=visionOS\ Simulator,OS=1.2,name=Apple\ Vision\ Pro, build)	1s
test (platform=macOS, test)	1m 34s
xcode-test-5_5	1s
cocoapods	1s
docs	1s
	21m 37s

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