chore: update phpunit/phpunit 8.5.52

[ewaostrowska]

The changes mainly focus on updating PHPUnit usage in generated test files and clarifying PHP version requirements.

PHPUnit compatibility and modernization:

• Updated the base test class in api_test.mustache and model_test.mustache to use \PHPUnit\Framework\TestCase instead of the deprecated \PHPUnit_Framework_TestCase, and added type hints to setup and teardown methods for better code quality and compatibility with PHPUnit 8+.
• Updated the development dependency for PHPUnit in composer.mustache from version ^4.8 to ^8.5.52, ensuring tests run with a more recent PHPUnit version.

Documentation update:

• Added a note to the README.mustache clarifying that running tests requires PHP 7.2 or later, even though the runtime library supports PHP 5.5 and later.