feat: bump Firebase iOS SDK to `11.8.0` #817

Job	Run time
ios (tests)	20m 28s
ios (tests)	21m 37s
ios (tests)	15m 24s
ios (packages/cloud_firestore/cloud_firestore/example)	10m 49s
ios (packages/cloud_firestore/cloud_firestore/example)	10m 49s
ios (packages/cloud_firestore/cloud_firestore/example)	10m 49s
	1h 29m 56s

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