serdect: bump `toml` to v0.8 #1956

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	4m 40s
test (1.60.0)	5m 12s
build (1.60.0, wasm32-unknown-unknown)	4m 37s
test (stable)	1m 1s
build (stable, thumbv7em-none-eabi)	14s
build (stable, wasm32-unknown-unknown)	20s
minimal-versions / minimal-versions	1m 31s
	17m 35s

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