serdect: bump `toml` to v0.8 #1957

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	5m 40s
build (1.60.0, wasm32-unknown-unknown)	4m 52s
test (1.60.0)	6m 10s
build (stable, thumbv7em-none-eabi)	22s
test (stable)	49s
build (stable, wasm32-unknown-unknown)	11s
minimal-versions / minimal-versions	1m 15s
	19m 19s

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