Bump serde from 1.0.210 to 1.0.215 #2202

Job	Run time
check	2m 36s
rustfmt	4s
clippy	3m 49s
integration-test-althea-payment	16m 4s
integration-test-debts	13m 44s
audit	17s
integration-test-five-nodes	10m 44s
test	5m 41s
integration-test-db-migration	11m 15s
integration-test-eth-payments	13m 31s
integration-test-multi-exit	10m 57s
integration-test-validate-contract	10m 16s
cross-aarch64	4m 32s
cross-armv7	4m 18s
	1h 47m 48s

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