Update @todo comment in \Drupal\Core\Routing\UrlGenerator::getRouteStringIdentifier

Problem/Motivation

In #3123832: [META] Fix @todo items referencing closed issues we've discovered there's a @todo in \Drupal\Core\Routing\UrlGenerator::getRouteStringIdentifier.
This @todo is linked to an d.o. issue that is already closed: #3151019: Only allow route names, deprecate support for route objects in UrlGenerator methods.

This is the @todo:

   * @todo Remove in https://www.drupal.org/i/3151019

Steps to reproduce

Proposed resolution

#3151019: Only allow route names, deprecate support for route objects in UrlGenerator methods was closed, but has a follow-up issue for cleanup in D11: #3339710: Disallow passing route objects to UrlGenerator methods

Let's replace the link in the @todo with the latter one.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet