release_config_lib: teach redacted print to respect --quiet

When running `build-flag --quiet`, the tool still prints information
about redacted flags. Change the print calling printf to calling warnf,
which will skip printing to stdout in case --quiet was used.

Bug: 384438973
Test: build-flag --quiet get --all | head # no "Redacting flag" lines
Change-Id: I101309fa26994522de16082a67cb5acca76c78d7
diff --git a/cmd/release_config/release_config_lib/flag_artifact.go b/cmd/release_config/release_config_lib/flag_artifact.go
index cb13fdc..f493e1e 100644
--- a/cmd/release_config/release_config_lib/flag_artifact.go
+++ b/cmd/release_config/release_config_lib/flag_artifact.go
@@ -189,7 +189,7 @@
 	if redacted {
 		fa.Redact()
 		flagValue.proto.Value = fa.Value
-		fmt.Printf("Redacting flag %s in %s\n", name, flagValue.path)
+		warnf("Redacting flag %s in %s\n", name, flagValue.path)
 	} else {
 		// If we are assigning a value, then the flag is no longer redacted.
 		fa.Redacted = false