Update plugin to be compatible with PHP 8.3

[Copilot]

The plugin's dependencies and method signatures were incompatible with PHP 8.3.

Changes

• PHP version constraint: ^7.4 || ^8.0 || ^8.1 → ^7.4 || ^8.0 (covers all 8.x)
• Method signature mismatches: QueueInterface declared non-nullable params for set_entries, set_entry_handler, set_entry_fetcher but implementations used nullable types — generates deprecation notices in PHP 8.3
• Dev dependencies: Replaced pinned/incompatible packages (predis 2.0.x-dev, phpunit ^9, geniem-rules-codesniffer, composer-git-hooks) with PHP 8.3-compatible versions
• Null safety: Base::set_entries() and MockQueue::set_entries() now handle null input correctly

Interface fix example

// Before (incompatible signatures between interface and implementation)
// QueueInterface:
public function set_entries( array $entries );
// Base.php:
public function set_entries( ?array $entries );  // PHP 8.3 deprecation

// After (interface updated to match legitimate nullable usage)
public function set_entries( ?array $entries );

All 11 tests pass on PHP 8.3 with zero deprecation notices.