Improve composer details, security and performance

[JoosuaKoskinen]

Makes the following changes to composer.json:

• Fix license to match the LICENSE file. License changed from MIT to GPLv3 in https://github.com/Seravo/wordpress/commit/68e404abfa7ab44231f3fb9e79e9f37b3da231ea.
• Add "support" links to GitHub and docs.seravo.com.
• Limit repository scopes to harden security.
• Disable platform checks for performance. It was useless as we don't want to prevent the site from working with PHP < 7.2.
• Optimize autoloader for performance. Non-optimized autoloader isn't meant for production use.