Add PHP version check for plugin.

[rahulsprajapati]

Description of the Change

Add minimum version check for the plugin before loading it. This will ensure plugin update doesn't break the site that don't match our minimum PHP version.

Closes #541

How to test the Change

1. Setup a WordPress environment running PHP 7.3
2. Install ClassifAI plugin by downloading it with this feature branch.
3. Verify that plugin functionality doesn't load and you see an admin error message for php version. ( refer above screenshot in Description )

Changelog Entry

Fixed - Better error handling for environments that don't match our minimum PHP version

Credits

Props @dkotter, @rahulsprajapati

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.