Switch from mitchellh/cli to hashicorp/cli (#24239)

@mitchellh suggested we fork `cli` and switch to that.

Since we primarily use the interfaces in `cli`, and the new
fork has not changed those, this is (mostly) a drop-in replacement.

A small fix will be necessary for Vault Enterprise, I believe.

command/ssh_test.go

@@ -7,7 +7,7 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/mitchellh/cli"
+	"github.com/hashicorp/cli"
 )
 
 func testSSHCommand(tb testing.TB) (*cli.MockUi, *SSHCommand) {