Add phpunit 12 support and drop phpunit 9 and 10

[lennartvava]

Summary

Modernizes district09/qa-php for PHP 8.3–8.5 and Drupal 11/12-era QA tooling.

Changes

• Releases the package as the breaking 3.0.0 line:
  • Requires PHP ^8.3.
  • Supports PHPUnit ^11.5 || ^12.5.
  • Removes legacy PHPUnit 9 and 10 support.
• Adds PHPUnit-version-aware configuration generation:
  • PHPUnit 11 receives a schema-correct phpunit-11.xml.
  • PHPUnit 12 receives the modernized default phpunit.xml.
  • Consumers continue using the generated phpunit.qa-php.xml file.
• Preserves PHPCPD checks while allowing both PHPUnit lines:
  • Composer resolves PHPCPD 8.0.0 with PHPUnit 11.
  • Composer resolves PHPCPD 8.3.0 with PHPUnit 12.
• Refreshes compatible QA dependencies, while retaining PHPCS 3 and Slevomat 8.22 because PHPCompatibility 9 does not yet support PHPCS 4.
• Replaces Travis CI with GitHub Actions:
  • Full QA run on PHP 8.3 / PHPUnit 12.
  • PHPUnit matrix for versions 11.5 and 12.5 on PHP 8.3, 8.4, and 8.5.
  • Enables Xdebug for coverage-report generation.
• Adds tests for PHPUnit configuration selection and unsupported-version handling.
• Updates README, changelog, PHP compatibility guidance, and generated-artifact ignores.

Verification

• PHPUnit 11.5: passes with the generated PHPUnit 11.5 configuration.
• PHPUnit 12.5 on PHP 8.3: passes with the generated PHPUnit 12.5 configuration.
• PHPUnit 12.5 on PHP 8.5: passes.
• Composer validation and normalization pass.
• PHPCS, PHPMD, PHPStan, PHPCPD, PHPUnit, security checks, and YAML validation pass.