serdect: bump `toml` to v0.8 #334

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 42s
build (1.65.0, wasm32-unknown-unknown)	5m 47s
build (stable, thumbv7em-none-eabi)	41s
build (stable, wasm32-unknown-unknown)	31s
test (1.65.0)	5m 8s
test (stable)	36s
minimal-versions / minimal-versions	1m 5s
	19m 30s

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