doc: V8 branch used in 8.x not active anymore

Add 8.x to the LTS versions that use a V8 branch that is
not active anymore.

PR-URL: https://github.com/nodejs/node/pull/18155


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>