input: Write a python function to replace all occurrences of a character in a string with another character.
  assert replace_all_occurrences("Hello World!", 'l', 'X') == "HeXXo WorXd!"
  assert replace_all_occurrences("abcdefg", 'a', 'z') == "zbcdefg"
  assert replace_all_occurrences("123123123", '1', '9') == "923923923"
output: def replace_all_occurrences(input_string, old_char, new_char):
      return input_string.replace(old_char, new_char)