v6.3: Update for full compatibility with zeitwerk

[danigirl329]

• Removes the gem loader for extensions as this has been deprecated for a while
• Rewrites the module MultiSite::ScopedValidation to use prepend methods as this was causing a stack error when making changes and reloading
• Comment out dependency observer. This should no longer be needed for zeitwerk should be autoloading extensions and dependencies.
• Shows that in the new world, config.to_prepare should be utilized instead of after_initialize. This file makes no difference but when the engine is installed this configuration will be passed on to the app that is installing trusty-cms. The .to_prepare will direct the app to reload what is inside this block, so instead of using the dependency observer, the app will deactivate and reactive extensions inside this block instead.
• Register v6.3 on rubygems