elliptic-curve: fix `hkdf` version requirement and test in CI (#1353) #1684

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	3m 30s
build (1.56.0, wasm32-unknown-unknown)	4m 22s
minimal-versions	37s
build (stable, thumbv7em-none-eabi)	20s
test (1.56.0)	4m 20s
test (stable)	13s
build (stable, wasm32-unknown-unknown)	12s
	13m 34s

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