input: Write a python function to convert all strings in a list to uppercase.
  assert convert_to_uppercase(['apple', 'banana', 'cat', 'dog', 'elephant'])==['APPLE', 'BANANA', 'CAT', 'DOG', 'ELEPHANT']
  assert convert_to_uppercase(['hello', 'world'])==['HELLO', 'WORLD']
  assert convert_to_uppercase(['python', 'rocks'])==['PYTHON', 'ROCKS']
output: def convert_to_uppercase(strings):
      return [string.upper() for string in strings]