ecdsa: adopt `der` API changes #1117

Job	Run time
test (1.85.0)	19s
test (stable)	18s
build (thumbv7em-none-eabi, stable)	14s
build (wasm32-unknown-unknown, stable)	18s
build (thumbv7em-none-eabi, 1.85.0)	16s
build (wasm32-unknown-unknown, 1.85.0)	18s
	1m 43s

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