Wrong check when trying to build legacy OSes on pre-15.6 macOS

[FurloSK]

Attempt to build legacy macOS images (High Sierra, Mojave, and Catalina) even on e.g. macOS Mojave fails with an error:

*** Error: building High Sierra, Mojave, and Catalina installer images
*** is unsupported on macOS Sequoia 15.6 and later due to breaking changes
*** in /usr/sbin/installer by Apple.

I believe commit 40ae77b has broken building by removing the original condition (of previous commit efe68b8) on lines #257-258:

if is_legacy:
  if is_15_6_or_later:
    print('*** Error: building High Sierra, Mojave, and Catalina installer images')
    print('*** is unsupported on macOS Sequoia 15.6 and later due to breaking changes')
    print('*** in /usr/sbin/installer by Apple.')
  return False

with condition on line #651 which no longer checks whether the macOS is new (that is, 15.6+):

if is_legacy(product_info[product_id].get('title','')):
  # Catalina and earlier do not have InstallAssistant.pkg, and
  # InstallAssistantAuto.pkg (which they do have) do not work for
  # installing the Install macOS.app
  print('*** Error: building High Sierra, Mojave, and Catalina installer images', file=sys.stderr)
  print('*** is unsupported on macOS Sequoia 15.6 and later due to breaking changes', file=sys.stderr)
  print('*** in /usr/sbin/installer by Apple.', file=sys.stderr)
  exit(1)