RenderElement and FormElement base plugin classes are deprecated in 10.3 and removed in 12.0

Problem/Motivation

The change record https://www.drupal.org/node/3436275 → must be implemented.

Steps to reproduce

Proposed resolution

Remaining tasks

Wait with the implementation, since we have installed codebase with D9 active.
The new classes do not exist before D10.3, and will break updates of gthis module on older versions.

User interface changes

API changes

Data model changes