elliptic-curve: bump `crypto-bigint` to v0.6.0-pre.12 #2264

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	23s
build (1.71.0, wasm32-unknown-unknown)	24s
derive (1.71.0)	19s
build (stable, thumbv7em-none-eabi)	22s
derive (stable)	9s
build (stable, wasm32-unknown-unknown)	14s
doc	34s
test (1.71.0)	25s
test (stable)	21s
minimal-versions / minimal-versions	44s
	3m 55s

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