[Server] Rework UMLDIOperationHandlerRegistry

As mentioned in this class, this was implemented as an alternative to DIOperationHandlerRegistry, which required to have 1 CreateOperationHandler per Element Type to create. DIOperationHandlerRegistry doesn't have this restriction anymore, so we should be able to use it in a standard way, and remove our custom UMLDIOperationHandlerRegistry

Start by reading UMLDIOperationHandlerRegistry and DIOperationHandlerRegistry, especially the class comment describing why the custom registry was introduced. Confirm that the standard registry now supports the required operation handlers, then remove the custom registry and update its server usage so the project uses DIOperationHandlerRegistry directly.

Written by the indexing model from the issue text.