Update minimum iOS deployment target to 15 #7459

Job	Run time
prepare_matrix	6s
local-integration-tests-linux-openssl	19m 59s
ubuntu-22.04-Release-x86-static	36m 22s
ubuntu-22.04-Debug-x64-static	34m 22s
ubuntu-22.04-Debug-x86-static	34m 18s
ubuntu-22.04-Release-x64-static	35m 2s
macos-14-Release-arm64-static	27m 16s
windows-latest-Debug-x86-dynamic	42m 44s
windows-latest-Release-x64-static	50m 0s
macos-14-Debug-arm64-static	17m 18s
	4h 57m 27s

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