Refactor the current schema generation test case in order to reuse the
same logic for schema generation with php 8 attributes.

Create a new schema attribute reader for extracting schema info from php
attributes.

 - Refactor unit tests in order to remove php 8.1 specific tests from
 older php versions
 - Conditionally remove or add code to static analysis based on the
 current php version
 - Update minimum phpunit version