stream: move _writableState.buffer to EOL

API was deprecated back in the 0.11 days.

PR-URL: https://github.com/nodejs/node/pull/31165


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>