Implement an upgrade of the codebase to use Slim Framework version 4, which includes the following changes:
- Updating to Slim version 4 and the corresponding AzuraCore updates.
- Making all objects PSR-7 Request/Response agnostic (and creating RequestHelper and ResponseHelper objects to interact with the PSR-7 Req/Resp objects).
- Moving to PHP-DI as a DI container and implementing its autowiring support.
* Fix access to an undefined property
* Fix usage of undefined variable
* Fix undefined class Throwable error
* Fix undefined class Throwable error
* Fix undefined class Throwable error
* Fix class not found error and remove unused class import
* Remove usage and instantiation of non-existent controller
* Remove implicit declaration of property that is not used