Consider renaming RendererInterface::renderPlain() to ::renderInIsolation()

Problem/Motivation

::renderPlain() is a weird function name.

Proposed resolution

Quoting #2450993-4: Rendered Cache Metadata created during the main controller request gets lost:

renderPlain() now actually means renderInIsolation() (i.e. start its own render stack) — it originally meant "turn this render array into a plain string, and don't care about anything else". But it did that by starting a new stack, and then restoring the old one. So the "getting a plain string" part was only a symptom, what it really is about, is rendering in isolation.

Remaining tasks

Do it.

User interface changes

None.

API changes

Rename RendererInterface::renderPlain() to ::renderInIsolation(), but with BC compatibility layer.

Data model changes

None.