Require PHP 7.1 and support PHP 8.4

[itsgoingd]

• This MR increases the minimum required PHP version from 5.6 to 7.1 and fixes all new deprecations in PHP 8.4.
• Originally motivated by resolving this PHP 8.4 deprecation, which requires the use of a PHP 7.1 syntax (or dropping the affected type-hints) - https://wiki.php.net/rfc/deprecate-implicitly-nullable-types
• This will only affect 0.3% of Clockwork users who use older PHP versions, none of them are even on Clockwork 5.x.
• Scheduled to release with Clockwork 5.3, before PHP 8.4 is generally available.
• More aggressive minimum PHP version bump is probably coming in Clockwork 6.