refactor: replace error-ignored cobra output functions (#1454)

Signed-off-by: Billy Zha <[email protected]>

cmd/oras/internal/option/remote.go

@@ -176,9 +176,9 @@ func (opts *Remote) Parse(cmd *cobra.Command) error {
 // optional cmd prompt.
 func (opts *Remote) readSecret(cmd *cobra.Command) (err error) {
 	if cmd.Flags().Changed(identityTokenFlag) {
-		cmd.PrintErrln("WARNING! Using --identity-token via the CLI is insecure. Use --identity-token-stdin.")
+		_, _ = fmt.Fprintln(cmd.ErrOrStderr(), "WARNING! Using --identity-token via the CLI is insecure. Use --identity-token-stdin.")
 	} else if cmd.Flags().Changed(passwordFlag) {
-		cmd.PrintErrln("WARNING! Using --password via the CLI is insecure. Use --password-stdin.")
+		_, _ = fmt.Fprintln(cmd.ErrOrStderr(), "WARNING! Using --password via the CLI is insecure. Use --password-stdin.")
 	} else if opts.secretFromStdin {
 		// Prompt for credential
 		secret, err := io.ReadAll(os.Stdin)

cmd/oras/root/discover.go

@@ -18,6 +18,7 @@ package root
 import (
 	"context"
 	"errors"
+	"fmt"
 
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/spf13/cobra"
@@ -84,7 +85,7 @@ Example - Discover referrers of the manifest tagged 'v1' in an OCI image layout
 			if cmd.Flags().Changed("output") {
 				switch opts.Format.Type {
 				case "tree", "json", "table":
-					cmd.PrintErrf("[DEPRECATED] --output is deprecated, try `--format %s` instead\n", opts.Template)
+					_, _ = fmt.Fprintf(cmd.ErrOrStderr(), "[DEPRECATED] --output is deprecated, try `--format %s` instead\n", opts.Template)
 				default:
 					return errors.New("output type can only be tree, table or json")
 				}