serdect: bump `toml` to v0.8 #2013

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	6m 13s
build (1.60.0, wasm32-unknown-unknown)	5m 15s
build (stable, thumbv7em-none-eabi)	24s
test (1.60.0)	4m 50s
build (stable, wasm32-unknown-unknown)	13s
test (stable)	16s
minimal-versions / minimal-versions	33s
	17m 44s

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