chore: mark `Signature` as deprecated #2192

Job	Run time
docs	31s
check no_std	44s
clippy	23s
features stable	2m 10s
features stable --all-targets	3m 20s
fmt	13s
miri --no-default-features	4m 44s
features nightly	1m 37s
miri	4m 36s
test ubuntu-latest stable --no-default-features	1m 18s
features nightly --all-targets	2m 47s
test ubuntu-latest stable	1m 14s
miri --all-features	4m 38s
wasm	36s
test ubuntu-latest nightly --no-default-features	58s
test ubuntu-latest nightly	51s
test ubuntu-latest 1.81 --no-default-features	34s
test ubuntu-latest 1.81	35s
test windows-latest stable --no-default-features	2m 32s
test windows-latest stable	2m 41s
test windows-latest nightly --no-default-features	1m 58s
test windows-latest nightly	2m 9s
test windows-latest 1.81 --no-default-features	1m 30s
test windows-latest 1.81	1m 26s
test ubuntu-latest 1.81 --features json	57s
test windows-latest 1.81 --features json	1m 32s
test ubuntu-latest nightly --all-features	1m 7s
test windows-latest nightly --all-features	2m 24s
ci success	1s
	50m 6s

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