[12.x] [meta] Set Drupal 12 platform and browser requirements at least six months before the release

🌱 [policy] Default to requiring the latest stable PHP release available when a new major version reaches beta Active has been signed off, which means Drupal 12 will require PHP 8.5. We can't require PHP 8.5 until we support it, which we don't fully yet because it hasn't been released, but could open a child issue postponed now.