Support only PHP 8.3

BREAKING CHANGES

• Only support PHP 8.3
  - Make classes read-only when possible
  • Add types to constants - Upgrade PHPUnit to v11.3
    • Use static versions of asserts and expectations of invocations
  • Change CI pipeline to use PHP 8.3
• Changes to AbstractFileLoaderFixture:
  • Add LoadMode enumeration to define load modes of files
  • Change getLoadMode to return a LoadMode enumeration value instead of a string
  • Change derived classes to conform with new getLoadMode definition
• Changes to ConnectionPurger:
  • Add PurgeMode enumeration to define purge modes
  • Remove setPurgeMode and getPurgeMode
    • The purge mode must be set in the constuctor with a PurgeMode enumeration value (which defaults to PurgeMode::Delete)
  • Remove InvalidConnectionPurgeModeException
• Update documentation