updated project linter-rules and enforced them

lib/widget/about/about.dart

@@ -8,12 +8,12 @@ class About extends StatelessWidget {
   Widget build(BuildContext context) {
     return Scaffold(
       appBar: AppBar(
-        title: const Text("Über diese App"),
+        title: const Text('Über diese App'),
       ),
       body: const Card(
         elevation: 1,
         borderOnForeground: true,
-        child: Text("Marianum Fulda"),
+        child: Text('Marianum Fulda'),
       ),
     );
   }