PHP_CodeSniffer 4.0

Support and requires PHP_CodeSniffer 4.0 (#29, #30)

• Require slevomat/coding-standard that uses PHPCS 4.0
• Replace PSR12.Files.FileHeader.SpacingAfterBlock with the new modular rules
  • See PHPCSStandards/PHP_CodeSniffer@c9c4d05 and the upgrade guide.
• Require PHP 7.4, the same as PHPCS now requires