Rename .htmltest.yaml to .htmltest.yml (#1985)

* Rename .htmltest.yaml to .htmltest.yml

This is almost nothing but htmltest expects this exact name by default, so when running the
binary locally, it avoids to have to bother with providing the name of the config file.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

* Add changelog

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

---------

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

.github/workflows/main.yml

@@ -255,7 +255,7 @@ jobs:
       - name: "Run htmltest"
         uses: wjdp/htmltest-action@master
         with:
-          config: .htmltest.yaml
+          config: .htmltest.yml
 
   build-historical-spec:
     name: "📖 Build the historical backup spec"