elliptic-curve: bump `crypto-bigint` to v0.6.0-rc.2 #2156

Job	Run time
no_std (1.75.0, thumbv7em-none-eabi)	12s
test (1.75.0)	14s
no_std (1.75.0, wasm32-unknown-unknown)	15s
test (stable)	20s
no_std (stable, thumbv7em-none-eabi)	13s
no_std (stable, wasm32-unknown-unknown)	14s
minimal-versions	0s
	1m 28s

Provide feedback