serdect: bump `toml` to v0.8 #2475

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	8m 42s
build (1.65.0, wasm32-unknown-unknown)	7m 23s
derive (1.65.0)	4m 44s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.65.0, sudo apt update && sudo apt install gcc-mult...	24m 24s
build (stable, thumbv7em-none-eabi)	2m 14s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	23m 26s
build (stable, wasm32-unknown-unknown)	1m 31s
derive (stable)	26s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.65.0)	28m 57s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	17m 11s
minimal-versions / minimal-versions	16m 30s
	2h 15m 28s

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