serdect: bump `toml` to v0.8 #890

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 11s
build (1.65.0, wasm32-unknown-unknown)	5m 30s
test (1.65.0)	5m 11s
build (stable, thumbv7em-none-eabi)	9s
test (stable)	40s
build (stable, wasm32-unknown-unknown)	17s
minimal-versions / minimal-versions	1m 15s
	16m 13s

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