serdect: bump `toml` to v0.8 #2553

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 18s
build (1.65.0, wasm32-unknown-unknown)	5m 11s
build (stable, thumbv7em-none-eabi)	42s
build (stable, wasm32-unknown-unknown)	53s
test (1.65.0)	5m 59s
test (stable)	59s
minimal-versions / minimal-versions	1m 52s
	20m 54s

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