Address @todo in EarlyRenderingControllerWrapperSubscriber

Problem/Motivation

In \Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber there is a comment:

@todo Remove in Drupal 9.0.0, by disallowing early rendering.

Is this still relevant? Are we still planning to disallow early rendering, or should we remove this todo comment?

Steps to reproduce

Proposed resolution

• Determine if this is still relevant.
• Review additional comments in the docblock, e.g. "this just exists to ease the transition to Drupal 8".

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet