doc: move Boron releases to LTS column

Move the LTS releases for Node 6 to the `LTS` column in the
corresponding `CHANGELOG_V6.md`.

PR-URL: https://github.com/nodejs/node/pull/10827


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>