fix: use SHA384 for ECDSA P384 #1002

Job	Run time
linting / govulncheck	13s
linting / golangci-lint	2m 44s
tests / Run (ubuntu-latest)	38s
tests / Run (macos-latest)	59s
tests / Run (windows-latest)	3m 16s
examples / client (ubuntu-latest)	10s
examples / multirepo (ubuntu-latest)	13s
examples / client (macos-latest)	28s
examples / repository (ubuntu-latest)	7s
examples / multirepo (macos-latest)	28s
examples / client (windows-latest)	1m 37s
examples / repository (macos-latest)	27s
examples / root-signing (ubuntu-latest)	17s
examples / multirepo (windows-latest)	2m 13s
examples / repository (windows-latest)	2m 32s
examples / root-signing (macos-latest)	29s
examples / root-signing (windows-latest)	2m 11s
examples / tuf-client-cli (ubuntu-latest)	10s
examples / tuf-client-cli (macos-latest)	31s
examples / tuf-client-cli (windows-latest)	2m 27s
	22m 10s

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