Remove support for PHP <8.2

[hschimpf]

✨ What’s new

• Require PHP 8.2+ – enables readonly properties, promoted params, better typing.
• New domain-specific exception set ActionNotImplementedException, InvalidMessageReceivedException, NoWorkerDefinedException, WorkerAlreadyDefinedException, WorkerNotDefinedException, TaskExecutionFailedException plus ParallelException value-object for serialising worker-side errors.
• Refactored Scheduler, Runner and Task for stricter types and reduced mutability.
• CI matrix now runs on 8.2 · 8.3 · 8.4.

🛠 Changed

• Scheduler::using() now throws WorkerAlreadyDefinedException if you try to re-register an existing worker with constructor parameters.
• Scheduler::runTask() re-throws worker exceptions as TaskExecutionFailedException (instead of a generic RuntimeException).
• Numerous PSR-12 / coding-style tidy-ups.

🗑 Removed

• GitHub Actions for PHP 8.0 / 8.1 (the v2.x series remains available for those runtimes).
• Generic RuntimeException branches – replaced with the new specific exceptions.

🏷 Versioning note

• Raising the minimum PHP version , therefore this will be released as v3.0.0.
• Users on PHP < 8.2 will stay on the v2.x line.

See CHANGELOG.md for full details and upgrading instructions.

🧪 Testing & compatibility

• Full test-suite passes on PHP 8.2, 8.3, 8.4.
• Composer will prevent install/update on unsupported runtimes, ensuring safe adoption.