refactor: Use built-in clear to zero byte slice (#2566)

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## Overview

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Replace the manual for-loop with a call to the built-in clear function
to set all elements of the slice to their zero value. This change
improves code readability and makes the intent more explicit.

This is the idiomatic approach for this operation since Go 1.21.

Signed-off-by: alex9long <alex9long@outlook.com>

Author: alex9long

pkg/signer/file/local.go

@@ -436,9 +436,7 @@ func fallbackDeriveKey(passphrase []byte, keyLen int) []byte {
 
 // zeroBytes overwrites a byte slice with zeros
 func zeroBytes(b []byte) {
-	for i := range b {
-		b[i] = 0
-	}
+	clear(b)
 }
 
 // getAddress returns the Ed25519 address of the signer.