chore: bump version to 1.4.0, refactor plugin updater

[soderlind]

This pull request updates the plugin to version 1.4.0 and refactors the update mechanism to use a shared GitHub updater class. The main change is replacing the custom inline plugin updater with a reusable class-github-updater.php, which simplifies maintenance and aligns with best practices. All references and documentation have been updated accordingly.

Updater refactor and codebase simplification:

• Removed the custom updater class GitHubPluginUpdater from src/php/Update/GitHubPluginUpdater.php and replaced it with a shared, minimal wrapper GitHubUpdater in the new class-github-updater.php, leveraging the yahnis-elsts/plugin-update-checker library. [1] [2]
• Updated vmfa-rules-engine.php to use the new GitHubUpdater::init() method for update checks, removed the old updater instantiation, and adjusted the initialization hook to plugins_loaded for better compatibility.

Versioning and documentation:

• Bumped the plugin version to 1.4.0 in vmfa-rules-engine.php, package.json, and readme.txt, and updated the stable tag. [1] [2] [3] [4]
• Added a changelog entry in both CHANGELOG.md and readme.txt describing the updater change for version 1.4.0. [1] [2]