Remove readFile() and reverseSlice() in favour of stdlib #1102

Job	Run time
examples / multirepo (ubuntu-latest)	22s
examples / client (ubuntu-latest)	37s
examples / repository (ubuntu-latest)	26s
examples / multirepo (macos-latest)	14s
examples / root-signing (ubuntu-latest)	40s
examples / client (macos-latest)	12s
examples / repository (macos-latest)	14s
examples / root-signing (macos-latest)	19s
examples / client (windows-latest)	1m 26s
examples / multirepo (windows-latest)	4m 8s
examples / repository (windows-latest)	2m 7s
examples / root-signing (windows-latest)	4m 16s
examples / tuf-client-cli (ubuntu-latest)	1m 18s
linting / golangci-lint	16s
tests / Run (ubuntu-latest)	42s
examples / tuf-client-cli (macos-latest)	17s
linting / govulncheck	38s
tests / Run (macos-latest)	34s
tests / Run (windows-latest)	5m 10s
examples / tuf-client-cli (windows-latest)	2m 38s
	26m 34s

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