elliptic-curve: bump `crypto-bigint` to v0.6.0-pre.12 #2608

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	42s
build (1.73.0, wasm32-unknown-unknown)	35s
build (stable, thumbv7em-none-eabi)	31s
build (stable, wasm32-unknown-unknown)	31s
doc	50s
minimal-versions	0s
test (1.73.0)	28s
test (stable)	22s
test (nightly)	29s
	4m 28s

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