input: Write a python function to find all strings in a list that contain a given substring, case-insensitively.
  assert find_matching_strings(['Google', 'Amazon', 'Facebook', 'apple'], 'oo') == ['Google']
  assert find_matching_strings(['Google', 'Amazon', 'Facebook', 'apple'], 'Am') == ['Amazon']
  assert find_matching_strings(['Google', 'Amazon', 'Facebook', 'apple'], 'ap') == ['apple']
output: def find_matching_strings(strings, substring):
      substring_lower = substring.lower()
      return [s for s in strings if substring_lower in s.lower()]