Add a note about Ubuntu 18 deprecation (#29527)

Co-authored-by: Dusko Dobranic <101048884+ddobranic@users.noreply.github.com>
Co-authored-by: mc <42146119+mchammer01@users.noreply.github.com>
Co-authored-by: skedwards88 <skedwards88@github.com>

content/actions/using-workflows/workflow-syntax-for-github-actions.md

@@ -793,11 +793,11 @@ strategy:
   fail-fast: false
   matrix:
     node: [13, 14]
-    os: [macos-latest, ubuntu-18.04]
+    os: [macos-latest, ubuntu-latest]
     experimental: [false]
     include:
       - node: 15
-        os: ubuntu-18.04
+        os: ubuntu-latest
         experimental: true
 ```
 {% endraw %}