Align CI with the PHP 8 requirement

[vanderlee]

What changed

• Replace the obsolete PHP 7-era Travis build with GitHub Actions.
• Test the declared PHP 8.0+ support across PHP 8.0 through 8.5.
• Allow Composer to resolve an appropriate maintained PHPUnit major for each runtime.
• Add a composer test script and strict Composer validation.
• Restore the shared PHPUnit test base alias, modernize the output data provider, and remove obsolete PHPUnit XML attributes.
• Use assertions compatible with PHPUnit 9 through 11.

Why

composer.json requires PHP 8.0, but the existing CI only targeted PHP 7.1, PHP 7.2, and HHVM. The old test bootstrap and fixtures also depended on PHPUnit APIs that changed across the supported runtime range.

Validation

The GitHub Actions matrix passes on PHP 8.0, 8.1, 8.2, 8.3, 8.4, and 8.5.