ci: bump syn from 2.0.118 to 3.0.3 #46

Job	Run time
miri (x86_64-unknown-linux-gnu)	34s
build (stable, wasm32-unknown-unknown)	20s
miri (s390x-unknown-linux-gnu)	54s
build (1.85.0, wasm32-unknown-unknown)	14s
build (1.85.0, thumbv7em-none-eabi)	13s
test (stable)	18s
build (stable, thumbv7em-none-eabi)	14s
test (1.85.0)	18s
minimal-versions / minimal-versions	25s
	3m 30s

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