Upgrade to `package:`objective_c` 4.0 #107

Job	Run time
health / changelog / run changelog health check	32s
health / breaking / run breaking health check	23s
health / coverage / run coverage health check	38s
health / do-not-submit / run do-not-submit health check	30s
health / leaking / run leaking health check	29s
health / license / run license health check	36s
health / comment	18s
	3m 26s

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