[Storage] Addressing Swift 6 issues with `Storage`'s instance management #8377

Job	Run time
changes	4s
spm-binary-cron	0s
check	0s
check-firestore-internal-public-headers	0s
cmake-prod-db	0s
pod-lib-lint	0s
cmake	0s
pod-lib-lint-cron	0s
sanitizers-mac	0s
sanitizers-ubuntu	0s
spm-binary	0s
spm-source	0s
xcodebuild	0s
Check all required Firestore tests results	1s
	5s

Provide feedback