Migrate CI to GitHub Actions and Support PHP 8.x

[DeepDiver1975]

This change migrates the entire CI/CD infrastructure from Drone to GitHub Actions.
It includes:

• Six new workflow files in .github/workflows/ covering all major test suites.
• Matrix support for PHP 7.4 through 8.5 across relevant workflows.
• Service containers with health checks for databases and other services.
• Update to ownCloud's core version check in lib/base.php to support PHP 8.x.
• Dependabot integration for GitHub Actions.
• Removal of the obsolete .drone.star configuration.

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PR created automatically by Jules for task 841417513468658110 started by @DeepDiver1975

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