doc: update changelog for v10.x LTS

PR-URL: #26931
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>

CHANGELOG.md

@@ -22,7 +22,7 @@ release.
 <table>
 <tr>
   <th title="Current"><a href="doc/changelogs/CHANGELOG_V11.md">11</a><sup>Current</sup></th>
-  <th title="Current"><a href="doc/changelogs/CHANGELOG_V10.md">10</a><sup>Current</sup></th>
+  <th title="LTS Until 2021-04"><a href="doc/changelogs/CHANGELOG_V10.md">10</a><sup>LTS</sup></th>
   <th title="LTS Until 2019-12"><a href="doc/changelogs/CHANGELOG_V8.md">8</a><sup>LTS</sup></th>
   <th title="LTS Until 2019-04"><a href="doc/changelogs/CHANGELOG_V6.md">6</a><sup>LTS</sup></th>
 </tr>