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: #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>

doc/guides/maintaining-V8.md

@@ -130,9 +130,10 @@ includes the following branches<sup>1</sup>:
 </table>
 
 
-The versions of V8 used in Node.js v4.x and v6.x have already been abandoned by
-upstream V8. However, Node.js needs to continue supporting these branches for
-many months (Current branches) or several years (LTS branches).
+The versions of V8 used in Node.js v4.x, v6.x, and 8.x have already been
+abandoned by upstream V8. However, Node.js needs to continue supporting
+these branches for many months (Current branches) or several
+years (LTS branches).
 
 # Maintenance Process