updated project linter-rules and enforced them

lib/api/mhsl/mhslApi.dart

@@ -15,21 +15,21 @@ abstract class MhslApi<T> extends ApiRequest {
   T assemble(String raw);
 
   Future<T> run() async {
-    Uri endpoint = Uri.parse("https://mhsl.eu/marianum/marianummobile/$subpath");
+    Uri endpoint = Uri.parse('https://mhsl.eu/marianum/marianummobile/$subpath');
 
     http.Response? data = await request(endpoint);
     if(data == null) {
-      throw ApiError("Request could not be dispatched!");
+      throw ApiError('Request could not be dispatched!');
     }
 
     if(data.statusCode > 299) {
-      throw ApiError("Non 200 Status code from mhsl services: $subpath: ${data.statusCode}");
+      throw ApiError('Non 200 Status code from mhsl services: $subpath: ${data.statusCode}');
     }
 
     return assemble(utf8.decode(data.bodyBytes));
   }
 
-  static String dateTimeToJson(DateTime time) => Jiffy.parseFromDateTime(time).format(pattern: "yyyy-MM-dd HH:mm:ss");
+  static String dateTimeToJson(DateTime time) => Jiffy.parseFromDateTime(time).format(pattern: 'yyyy-MM-dd HH:mm:ss');
   static DateTime dateTimeFromJson(String time) => DateTime.parse(time);
 
 }