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: #10827
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: addaleax

doc/changelogs/CHANGELOG_V6.md

@@ -7,13 +7,13 @@
 </tr>
 <tr>
 <td valign="top">
-  <a href="#6.9.4">6.9.4</a><br/>
-</td>
-<td valign="top">
+<a href="#6.9.4">6.9.4</a><br/>
 <a href="#6.9.3">6.9.3</a><br/>
 <a href="#6.9.2">6.9.2</a><br/>
 <a href="#6.9.1">6.9.1</a><br/>
 <a href="#6.9.0">6.9.0</a><br/>
+</td>
+<td valign="top">
 <a href="#6.8.1">6.8.1</a><br/>
 <a href="#6.8.0">6.8.0</a><br/>
 <a href="#6.7.0">6.7.0</a><br/>