serdect: bump `toml` to v0.8 #1087

Job	Run time
build (1.70.0, thumbv7em-none-eabi)	28s
test (1.70.0)	1m 48s
test (stable)	1m 56s
build (1.70.0, wasm32-unknown-unknown)	34s
build (stable, thumbv7em-none-eabi)	19s
build (stable, wasm32-unknown-unknown)	21s
minimal-versions / minimal-versions	2m 2s
	7m 28s

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