elliptic-curve: bump ff and group to pre-releases #2773

Job	Run time
test (stable)	12s
build (1.85.0, thumbv7em-none-eabi)	21s
test (1.85.0)	20s
build (stable, wasm32-unknown-unknown)	17s
build (stable, thumbv7em-none-eabi)	14s
build (1.85.0, wasm32-unknown-unknown)	18s
derive (1.85.0)	20s
derive (stable)	13s
minimal-versions	0s
	2m 15s

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