elliptic-curve: bump `crypto-bigint` to v0.6.0-rc.8; MSRV 1.83 #2974

Job	Run time
build (1.83.0, thumbv7em-none-eabi)	40s
minimal-versions	0s
build (1.83.0, wasm32-unknown-unknown)	39s
test (1.83.0)	29s
build (stable, thumbv7em-none-eabi)	41s
build (stable, wasm32-unknown-unknown)	42s
test (stable)	29s
test (nightly)	28s
	4m 8s

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