12.0.0: Drop PHP 7.4, add 8.4/8.5+

• Heads up: Dropped support for PHP < 7.4, see xp-framework/rfc#343
  (@thekid)
• Migrated to the xp-framework/test library - @thekid
• Fixed deprecation warning Non-canonical cast (double) is deprecated,
  use the (float) cast instead in PHP 8.5
  (@thekid)