elliptic-curve: bump ff and group to pre-releases #2020

Job	Run time
build (1.85.0, thumbv7em-none-eabi)	16s
build (1.85.0, wasm32-unknown-unknown)	18s
build (stable, wasm32-unknown-unknown)	22s
test (1.85.0)	22s
build (stable, thumbv7em-none-eabi)	11s
test (stable)	21s
minimal-versions / minimal-versions	38s
	2m 28s

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