doc: mark Node.js v15.x as EOL

Today is 2021-06-01, which is the date v15.x release line is marked as
End-of-Life.

Refs: https://github.com/nodejs/Release/blob/main/schedule.json

PR-URL: #38891
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Nitzan Uziely <linkgoron@gmail.com>

CHANGELOG.md

@@ -3,7 +3,7 @@
 Select a Node.js version below to view the changelog history:
 
 * [Node.js 16](doc/changelogs/CHANGELOG_V16.md) **Current**
-* [Node.js 15](doc/changelogs/CHANGELOG_V15.md) **Current**
+* [Node.js 15](doc/changelogs/CHANGELOG_V15.md) End-of-Life
 * [Node.js 14](doc/changelogs/CHANGELOG_V14.md) **Long Term Support**
 * [Node.js 13](doc/changelogs/CHANGELOG_V13.md) End-of-Life
 * [Node.js 12](doc/changelogs/CHANGELOG_V12.md) Long Term Support
@@ -27,7 +27,6 @@ release.
 <table>
 <tr>
   <th title="Current"><a href="doc/changelogs/CHANGELOG_V16.md">16</a><sup>Current</sup></th>
-  <th title="Current"><a href="doc/changelogs/CHANGELOG_V15.md">15</a><sup>Current</sup></th>
   <th title="LTS Until 2023-04"><a href="doc/changelogs/CHANGELOG_V14.md">14</a><sup>LTS</sup></th>
   <th title="LTS Until 2022-04"><a href="doc/changelogs/CHANGELOG_V12.md">12</a><sup>LTS</sup></th>
 </tr>
@@ -38,26 +37,6 @@ release.
 <a href="doc/changelogs/CHANGELOG_V16.md#16.0.0">16.0.0</a><br/>
     </td>
     <td valign="top">
-<b><a href="doc/changelogs/CHANGELOG_V15.md#15.14.0">15.14.0</a></b><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.13.0">15.13.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.12.0">15.12.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.11.0">15.11.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.10.0">15.10.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.9.0">15.9.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.8.0">15.8.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.7.0">15.7.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.6.0">15.6.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.5.1">15.5.1</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.5.0">15.5.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.4.0">15.4.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.3.0">15.3.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.2.1">15.2.1</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.2.0">15.2.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.1.0">15.1.0</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.0.1">15.0.1</a><br/>
-<a href="doc/changelogs/CHANGELOG_V15.md#15.0.0">15.0.0</a><br/>
-    </td>
-    <td valign="top">
 <b><a href="doc/changelogs/CHANGELOG_V14.md#14.16.1">14.16.1</a></b><br/>
 <a href="doc/changelogs/CHANGELOG_V14.md#14.16.0">14.16.0</a><br/>
 <a href="doc/changelogs/CHANGELOG_V14.md#14.15.5">14.15.5</a><br/>