Clarify which Feather icons aren't supported.

Also, document the current version of Feather icons at time of writing.
When additional icons are released in the future, including the version
of Feather at the time our documentation was last updated will help
developers realize that the newly added icons are not yet available.

Author: Kurt-von-Laven

content/actions/creating-actions/metadata-syntax-for-github-actions.md

@@ -479,11 +479,39 @@ The background color of the badge. Can be one of: `white`, `yellow`, `blue`, `gr
 
 ### `branding.icon`
 
-The name of the [Feather](https://feathericons.com/) icon to use.
+The name of the v4.28.0 [Feather](https://feathericons.com/) icon to use. Brand icons are omitted as well as the following:
+
+<table>
+<tr>
+<td>coffee</td>
+<td>columns</td>
+<td>divide-circle</td>
+<td>divide-square</td>
+</tr>
+<tr>
+<td>divide</td>
+<td>frown</td>
+<td>hexagon</td>
+<td>key</td>
+</tr>
+<tr>
+<td>meh</td>
+<td>mouse-pointer</td>
+<td>smile</td>
+<td>tool</td>
+</tr>
+<tr>
+<td>x-octagon</td>
+<td></td>
+<td></td>
+<td></td>
+</tr>
+</table>
+
+Here is an exhaustive list of all currently supported icons:
+
 <!-- 
   This table should match the icon list in `app/models/repository_actions/icons.rb` in the internal github repo.
-  This table does not match the latest version the feather library. 
-  (Brand icons are omitted, and our supported list is not necessarily up-to-date with the latest version of the feather icon library.)
   To support a new icon, update `app/models/repository_actions/icons.rb` and add the svg to `/static/images/icons/feather` in the internal github repo. 
 -->