fix: update OtpLength type to uint

Co-authored-by: Han Qiao <[email protected]>

pkg/config/config.go

@@ -304,7 +304,7 @@ type (
 
 	phoneFactorTypeConfiguration struct {
 		factorTypeConfiguration
-		OtpLength    int           `toml:"otp_length"`
+		OtpLength    uint          `toml:"otp_length"`
 		Template     string        `toml:"template"`
 		MaxFrequency time.Duration `toml:"max_frequency"`
 	}