chore: mark `Signature` as deprecated #2193

Job	Run time
miri --no-default-features	4m 58s
miri	5m 17s
test ubuntu-latest stable --no-default-features	1m 10s
clippy	44s
miri --all-features	7m 23s
test ubuntu-latest stable	1m 6s
check no_std	54s
test ubuntu-latest nightly --no-default-features	47s
test ubuntu-latest nightly	1m 32s
docs	32s
test ubuntu-latest 1.81 --no-default-features	34s
features stable	2m 7s
fmt	14s
test ubuntu-latest 1.81	35s
features stable --all-targets	3m 16s
wasm	30s
features nightly	1m 38s
test windows-latest stable --no-default-features	3m 33s
features nightly --all-targets	2m 50s
test windows-latest stable	3m 20s
test windows-latest nightly --no-default-features	1m 51s
test windows-latest nightly	1m 54s
test windows-latest 1.81 --no-default-features	1m 20s
test windows-latest 1.81	2m 12s
test ubuntu-latest 1.81 --features json	36s
test windows-latest 1.81 --features json	1m 44s
test ubuntu-latest nightly --all-features	1m 7s
test windows-latest nightly --all-features	2m 23s
ci success	1s
	56m 8s

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