chore(deps): use stdlib ed25519 instead of x #976

Job	Run time
examples / repository (ubuntu-latest)	12s
examples / client (ubuntu-latest)	9s
examples / repository (macos-latest)	16s
examples / client (macos-latest)	16s
examples / client (windows-latest)	1m 39s
examples / repository (windows-latest)	1m 45s
examples / multirepo (ubuntu-latest)	5s
examples / root-signing (ubuntu-latest)	9s
examples / root-signing (macos-latest)	24s
examples / multirepo (macos-latest)	28s
examples / root-signing (windows-latest)	1m 51s
examples / multirepo (windows-latest)	2m 2s
examples / tuf-client-cli (ubuntu-latest)	12s
linting / golangci-lint	13s
linting / govulncheck	27s
examples / tuf-client-cli (macos-latest)	36s
examples / tuf-client-cli (windows-latest)	2m 29s
tests / Run (ubuntu-latest)	1m 44s
tests / Run (macos-latest)	1m 3s
tests / Run (windows-latest)	2m 18s
	18m 18s

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