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

Problem/Motivation

According to this drupal core change-record:
RenderElement and FormElement base plugin classes are deprecated and renamed to RenderElementBase and FormElementBase →

Steps to reproduce

Proposed resolution

let us postpone this until D12 arrives, since we have still issues coming in from D9 users.

Remaining tasks

User interface changes

API changes

Data model changes