Migrate CI to GitHub Actions with a PHP 7.1–8.5 matrix

[Copilot]

This updates the project’s CI to a GitHub Actions-native pipeline and expands the runtime matrix to PHP 7.1 through 8.5. PHPUnit now runs on every supported version, while PHPStan runs on a single modern version to keep static analysis in the pipeline without multiplying job cost.

• CI pipeline

  • Reworked .github/workflows/ci.yml into two explicit jobs:
    • phpunit across PHP 7.1 → 8.5
    • phpstan on PHP 8.5
  • Updated workflow triggers and action versions
  • Switched to GitHub Actions-compatible Composer cache handling via $GITHUB_OUTPUT

• Dependency / toolchain compatibility

  • Standardized the workflow on Composer 2.2 for compatibility with older PHP runtimes in the matrix
  • Kept PHPStan installation scoped to the analysis job instead of coupling it to every test job

• PHPUnit config

  • Simplified phpunit.xml to remove legacy config that newer PHPUnit versions reject
  • Kept the test suite definition minimal so the same config can be used across the expanded matrix

• Runtime compatibility fix

  • Added an early return in PhoneticGerman when normalization strips the input down to an empty string
  • Avoids undefined-offset failures on newer PHP versions for inputs that become empty after transliteration/filtering

• Legacy CI cleanup

  • Removed obsolete Travis and CircleCI configs now that GitHub Actions is the single CI path

Example of the new matrix shape:

strategy:
  fail-fast: false
  matrix:
    php:
      - '7.1'
      - '7.2'
      - '7.3'
      - '7.4'
      - '8.0'
      - '8.1'
      - '8.2'
      - '8.3'
      - '8.4'
      - '8.5'