rename renderPlain to something by making renderPlain a wrapper and adding another better named method

Problem/Motivation

Follow-up to #2575615-81: Introduce HtmlEscapedText and remove SafeMarkup::setMultiple() and SafeMarkup::getAll() and remove the static safeStrings list →

\Drupal\Core\Render\RendererInterface::renderPlain
implies in its name that it will return plaintext, but returns html (in an object)

Proposed resolution

• come up with a better name
• move the code in \Drupal\Core\Render\Renderer::renderPlain to the new method
• change renderPlain to be a wrapper for the new method and deprecate renderPlain
• replace usages of renderPlain to the newly named method
• make a change record

Remaining tasks

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User interface changes

API changes

Data model changes