Migrate CI to GitHub Actions and extend coverage through PHP 8.5

[Copilot]

This replaces the Travis-based pipeline with GitHub Actions and updates the project’s CI tooling to run on current PHP releases. The workflow now validates the library through PHP 8.5 and aligns the test stack with modern GitHub-hosted runners.

• GitHub Actions pipeline

  • adds .github/workflows/ci.yml as the primary CI workflow
  • runs the test job across PHP 7.3 through PHP 8.5
  • keeps static analysis and coverage as separate jobs on the newest target
  • scopes workflow token permissions to read-only

• Tooling refresh

  • updates dev dependencies to supported CI-compatible versions:
    • phpunit/phpunit → ^9.6
    • phpstan/phpstan → ^1.12
  • migrates phpunit.xml to the PHPUnit 9 schema/config layout
  • adjusts PHPUnit assertions and lifecycle method signatures for newer PHPUnit releases

• Repository cleanup

  • removes .travis.yml
  • replaces Travis badges/references in README.md

jobs:
  tests:
    strategy:
      matrix:
        php-version: ['7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5']

  static-analysis:
    steps:
      - uses: shivammathur/setup-php@v2
        with:
          php-version: '8.5'

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