Replace fmt.Sprintf with hex.EncodeToString (#21960)

`hex.EncodeToString` has better performance than `fmt.Sprintf("%x",
[]byte)`, we should use it as much as possible.

I'm not an extreme fan of performance, so I think there are some
exceptions:

- `fmt.Sprintf("%x", func(...)[N]byte())`
- We can't slice the function return value directly, and it's not worth
adding lines.
    ```diff
    func A()[20]byte { ... }
    - a := fmt.Sprintf("%x", A())
    - a := hex.EncodeToString(A()[:]) // invalid
    + tmp := A()
    + a := hex.EncodeToString(tmp[:])
    ```
- `fmt.Sprintf("%X", []byte)`
- `strings.ToUpper(hex.EncodeToString(bytes))` has even worse
performance.

models/user/user.go

@@ -401,7 +401,7 @@ func hashPassword(passwd, salt, algo string) (string, error) {
 		tempPasswd = pbkdf2.Key([]byte(passwd), saltBytes, 10000, 50, sha256.New)
 	}
 
-	return fmt.Sprintf("%x", tempPasswd), nil
+	return hex.EncodeToString(tempPasswd), nil
 }
 
 // SetPassword hashes a password using the algorithm defined in the config value of PASSWORD_HASH_ALGO