Fix PHP 8.1/8.4 deprecations

[ddegasperi]

Addresses a series of deprecation warnings and compatibility issues introduced in PHP 8.1 and 8.4, and
adds GitHub Actions CI workflows.

PHP 8.4

• Fix implicit nullable parameter type in ezcPersistentSaveHandler::saveInternal() by adding explicit
  ? prefix

PHP 8.1

• Fix return type covariance issues on Iterator and ArrayObject subclasses
• Fix return types on additional ArrayObject subclasses
• Add void return types to test setUp/tearDown methods

Tests

• Fix deprecated PHPUnit API usage in tests

CI

• Add GitHub Actions workflows for CI and coding standard checks, using the shared
  zetacomponents/.github reusable workflows