Use strings.CutPrefix for cleaner ARN prefix stripping

Replace strings.HasPrefix checks with strings.CutPrefix for more
idiomatic Go code. This function is available in Go 1.20+ and
provides cleaner conditional logic with the combined check and
extraction.

Addresses review feedback from Gemini Code Assist.

weed/s3api/s3api_bucket_policy_handlers.go

@@ -293,10 +293,10 @@ func (s3a *S3ApiServer) validateResourceForBucket(resource, bucket string) bool
 	const seaweedPrefix = "arn:seaweed:s3:::"
 
 	// Strip the optional ARN prefix to get the resource path
-	if strings.HasPrefix(resource, awsPrefix) {
-		resourcePath = resource[len(awsPrefix):]
-	} else if strings.HasPrefix(resource, seaweedPrefix) {
-		resourcePath = resource[len(seaweedPrefix):]
+	if path, ok := strings.CutPrefix(resource, awsPrefix); ok {
+		resourcePath = path
+	} else if path, ok := strings.CutPrefix(resource, seaweedPrefix); ok {
+		resourcePath = path
 	} else {
 		resourcePath = resource
 	}