Bump `heapless` dependency to v0.8 #2116

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	5m 29s
test (1.60.0)	3m 54s
build (1.60.0, wasm32-unknown-unknown)	3m 40s
test (stable)	29s
build (stable, thumbv7em-none-eabi)	19s
build (stable, wasm32-unknown-unknown)	21s
derive (1.60.0)	5m 17s
derive (stable)	19s
doc	10s
minimal-versions / minimal-versions	25s
	20m 23s

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