[Symfony 5] Deprecate app.root and site.path services and either access DrupalKernal->getAppRoot and ->getSitePath directly, or add other suitable alternative.

Problem/Motivation

Using non-object services is deprecated as of Symfony 4.4. We create app.root and site.path services as strings in our services file

Proposed resolution

We should deprecate these two string services, and either use the kernel service directly, or provide some other alternative service to access those strings.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet