elliptic-curve: bump ff and group to pre-releases (#1821) #2475

Job	Run time
test (1.85.0)	20s
no_std (stable, thumbv7em-none-eabi)	9s
test (stable)	9s
no_std (1.85.0, thumbv7em-none-eabi)	17s
no_std (1.85.0, wasm32-unknown-unknown)	14s
no_std (stable, wasm32-unknown-unknown)	12s
minimal-versions	0s
	1m 21s

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