Add deprecated warning, output looks like the following:

```
$ gaiacli keys update --help
Command "update" is deprecated, it takes no effect with the new keyring
based backend and is provided only for backward compatibility with the
legacy LevelDB based backend.
Refer to your operating system's manual to learn how to change your
keyring's password.

Change the password used to protect private key

Usage:
  gaiacli keys update <name> [flags]

Flags:
  -h, --help   help for update

Global Flags:
      --chain-id string   Chain ID of tendermint node
  -e, --encoding string   Binary encoding (hex|b64|btc) (default "hex")
      --home string       directory for config and data (default "/home/alessio/.gaiacli")
  -o, --output string     Output format (text|json) (default "text")
      --trace             print out full stack trace on errors
```

client/keys/update.go

@@ -12,8 +12,14 @@ func updateKeyCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "update <name>",
 		Short: "Change the password used to protect private key",
-		RunE:  runUpdateCmd,
-		Args:  cobra.ExactArgs(1),
+		Deprecated: `it takes no effect with the new keyring
+based backend and is provided only for backward compatibility with the
+legacy LevelDB based backend.
+Refer to your operating system's manual to learn how to change your
+keyring's password.
+`,
+		RunE: runUpdateCmd,
+		Args: cobra.ExactArgs(1),
 	}
 	return cmd
 }