.codecov.yml: enforce 90% minimum coverage #416

Job	Run time
careful	59s
clippy	22s
coverage	18s
doc	21s
rustfmt	5s
miri (s390x-unknown-linux-gnu)	1m 2s
test (stable)	43s
build (1.85.0, thumbv7em-none-eabi)	1m 19s
build (1.85.0, wasm32-unknown-unknown)	1m 17s
test (1.85.0)	49s
build (stable, armv7a-none-eabi)	1m 14s
build (stable, wasm32-unknown-unknown)	1m 7s
build (stable, thumbv7em-none-eabi)	1m 15s
build (1.85.0, armv7a-none-eabi)	1m 22s
miri (x86_64-unknown-linux-gnu)	47s
	13m 0s

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