Add note to issue template regarding supported OS

This commit adds a note to the GitHub issue template noting that bug
reports on OS that we do not support or feature requests for OS that we
do not support will be closed.

Relates elastic#19322

.github/ISSUE_TEMPLATE.md

@@ -3,7 +3,11 @@ GitHub is reserved for bug reports and feature requests. The best place
 to ask a general question is at the Elastic Discourse forums at
 https://discuss.elastic.co. If you are in fact posting a bug report or
 a feature request, please include one and only one of the below blocks
-in your new issue.
+in your new issue. Note that whether you're filing a bug report or a
+feature request, ensure that your submission is for an
+[OS that we support](https://www.elastic.co/support/matrix#show_os).
+Bug reports on OS that we do not support or feature requests specific
+to OS that we do not support will be closed.
 -->
 
 <!--