Add PHP 7.4 support and set as default

[Copilot]

This PR adds support for PHP 7.4 to the plugin-tester Docker images and sets it as the new default version for both the latest tag and WordPress-only tags.

Changes Made

Added PHP 7.4 Support:

• Created Dockerfile.php7.4 using Alpine 3.13 with PHP 7.4 packages
• Updated GitHub Actions workflow to include PHP 7.4 in the build matrix
• Now builds 45 total image combinations (9 WordPress versions × 5 PHP versions)

Set PHP 7.4 as Default:

• Changed the latest tag to use WordPress 6.8 with PHP 7.4 (previously PHP 8.0)
• Updated WordPress-only tags (e.g., wp-6.8, wp-6.7) to default to PHP 7.4
• Users can still access specific PHP versions using explicit tags like wp-6.8-php8.3

Updated Documentation:

• Updated README to reflect PHP 7.4 support and new default behavior
• Changed examples to show PHP 7.4 as the default
• Maintained clear documentation about available PHP versions (7.4, 8.0, 8.1, 8.2, 8.3)

Usage Examples

# Uses latest WordPress (6.8) with PHP 7.4 (new default)
docker run --rm -v "$PWD:/code" humanmade/plugin-tester

# WordPress-only tag now defaults to PHP 7.4
docker run --rm -v "$PWD:/code" humanmade/plugin-tester:wp-6.8

# Specific PHP versions still available
docker run --rm -v "$PWD:/code" humanmade/plugin-tester:wp-6.8-php8.3

The implementation maintains full backward compatibility - all existing specific version tags continue to work exactly as before. Only the default behavior has changed to use PHP 7.4 instead of PHP 8.0.

Fixes #26.

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