refactor(admin): Correct extension of HTML twig template

i.e. ".twig" to ".html.twig"

Author: pan93412

src/Controller/Admin/EmailTemplateController.php

@@ -34,7 +34,7 @@ public function index(): Response
             $templateFiles
         );
 
-        return $this->render('admin/email_template/index.twig', [
+        return $this->render('admin/email_template/index.html.twig', [
             'templates' => $templateFiles,
         ]);
     }
@@ -57,7 +57,7 @@ public function details(string $name, Request $request): Response
             $error = $e->getMessage();
         }
 
-        return $this->render('admin/email_template/details.twig', [
+        return $this->render('admin/email_template/details.html.twig', [
             'name' => $name,
             'parameters' => $parameters,
             'content' => $content,