elliptic-curve: minimum supported `serde_json` is v1.0.47 (#1354) #1259

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	4m 50s
build (1.56.0, wasm32-unknown-unknown)	3m 30s
test (1.56.0)	3m 34s
build (stable, thumbv7em-none-eabi)	10s
test (stable)	22s
build (stable, wasm32-unknown-unknown)	22s
minimal-versions / minimal-versions	26s
	13m 14s

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