Issue #3580405: Retarget renderEmbedCode() removal from 3.2.0 to 4.0.0 (SemVer alignment)

Follow-up to merged !114.

Per SemVer, removing a public method between minor releases (3.1 → 3.2) is a breaking change that should be a major bump. Updates the @deprecated block, the @trigger_error() calls and the @todo comments to point to 4.0.0 as the removal target instead of 3.2.0.

No behavior change — pure docblock / message text updates.

Thanks @kmonty for the SemVer pushback in #3580405-comment-16568989.