UrlGenerator should either inject RequestContext or explain why it's not doing it

Problem/Motivation

UrlGenerator::__construct() creates its own RequestContext object, but this is also available as a service.

Is there a reason it's doing this? if so, it should say why.

If there is no reason, it should have the service injected.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet