keys: rename `show` to `secret` #2118

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	13m 50s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	15m 55s
build (stellar-cli, stellar, macos-14, aarch64-apple-darwin)	9m 8s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	8m 44s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	19m 52s
installer	57s
	1h 8m 26s

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