Run htmlvalidator on *.htm files too

Taken from
eclipse-platform#3342

Author: akurtakov

.github/workflows/htmlvalidator.yml

@@ -14,4 +14,5 @@ jobs:
       uses: Cyb3r-Jak3/html5validator-action@v8.0.0
       with:
         root: /github/workspace/eclipse.platform.common/bundles
-        blacklist: about.html notices.html
+        blacklist: about.html notices.html
+        extra: --match *.html  --match *.htm