Drop Support for Older PHP Versions & Fix Version Mismatches

[ashleyshaw]

Description
Align the minimum supported PHP version to at least 8.1, and fix any version requirement inconsistencies in composer.json, plugin readme, and PHPCS configuration.

Why This Matters

• PHP 7.4 is EOL; many modern PHP tools don’t support it.
• Consistency in version requirements helps avoid confusion and dependency issues.

Tasks

1. Update composer.json, plugin readme, and PHPCS to require PHP >= 8.1.
2. Remove or refactor code tailored to PHP < 8.1.
3. Validate and test against PHP 8.1 (and optionally 8.2+ for future-proofing).