elliptic-curve: bump `crypto-bigint` to v0.6.0-pre.8; MSRV 1.73 (#1463) #1481

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 43s
build (1.65.0, wasm32-unknown-unknown)	2m 41s
test (1.65.0)	1m 46s
build (stable, thumbv7em-none-eabi)	15s
test (stable)	21s
build (stable, wasm32-unknown-unknown)	19s
minimal-versions / minimal-versions	31s
	7m 36s

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