Add migration guide for 1.16. (#3491) #6111

Job	Run time
Library (evolution)	5m 51s
Examples	27m 7s
Check Macro Compatibility	3m 14s
xcodebuild (15) (test, IOS, 15.4)	54m 29s
xcodebuild (16) (IOS, 16.0)	10m 22s
xcodebuild (15) (test, MAC_CATALYST, 15.4)	35m 56s
xcodebuild (16) (MACOS, 16.0)	4m 20s
xcodebuild (15) (test, MACOS, 15.4)	29m 10s
xcodebuild (15) (test, TVOS, 15.4)	1h 1m 31s
xcodebuild (15) (test, VISIONOS, 15.4)	1h 57m 44s
xcodebuild (15) (test, WATCHOS, 15.4)	48m 43s
xcodebuild (15) (IOS, 15.2)	13m 39s
xcodebuild (15) (MACOS, 15.2)	5m 21s
	6h 57m 27s

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