srp: bump `crypto-bigint` to v0.7 (#279) #281

Job	Run time
test (stable)	2m 13s
build (1.85, wasm32-unknown-unknown)	17s
build (stable, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	19s
build (1.85, thumbv7em-none-eabi)	17s
test (1.85)	2m 27s
	5m 52s

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