Add migration guide for 1.16. #6101

Job	Run time
Library (evolution)	4m 15s
Check Macro Compatibility	3m 21s
Examples	14m 48s
xcodebuild (15) (test, IOS, 15.4)	6m 50s
xcodebuild (15) (test, MAC_CATALYST, 15.4)	3m 53s
xcodebuild (16) (IOS, 16.0)	8m 27s
xcodebuild (15) (test, MACOS, 15.4)	4m 15s
xcodebuild (16) (MACOS, 16.0)	4m 21s
xcodebuild (15) (test, TVOS, 15.4)	16m 30s
xcodebuild (15) (test, VISIONOS, 15.4)	12m 58s
xcodebuild (15) (test, WATCHOS, 15.4)	50m 28s
xcodebuild (15) (IOS, 15.2)	7m 25s
xcodebuild (15) (MACOS, 15.2)	5m 24s
	2h 22m 55s

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