fix: correct `encode_2718_len` for legacy transactions (#1360) #4170

Job	Run time
doctest	2m 38s
feature-checks	6m 46s
wasm-unknown	1m 46s
test ubuntu-latest stable --no-default-features	2m 26s
wasm-wasi	2m 40s
test ubuntu-latest stable	2m 4s
test ubuntu-latest stable --all-features	2m 41s
cfg-check	35s
test ubuntu-latest nightly --no-default-features	1m 21s
test ubuntu-latest nightly	1m 33s
test ubuntu-latest nightly --all-features	2m 8s
check no_std	35s
test ubuntu-latest 1.79 --no-default-features	42s
test ubuntu-latest 1.79	42s
clippy	1m 9s
test windows-latest stable --no-default-features	4m 46s
test windows-latest stable	4m 48s
docs	1m 9s
test windows-latest stable --all-features	6m 45s
test windows-latest nightly --no-default-features	3m 54s
fmt	13s
test windows-latest nightly	5m 15s
test windows-latest nightly --all-features	4m 55s
test windows-latest 1.79 --no-default-features	2m 42s
test windows-latest 1.79	3m 38s
	1h 7m 51s

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