[Symfony 5] Deprecate app.root service and replace with a container parameter.

Problem/Motivation

Using non-object services is deprecated as of Symfony 4.4. We create the app.root services as a sting in our services file.

Proposed resolution

Deprecate app.root as a service, and replace with a container parameter with the same name.

Remaining tasks

Determine how to deal with an app.root that has changed while we still have a cached container definition.

User interface changes

API changes

Data model changes

Release notes snippet