Replace deprecated withOpacity with withValues

Author: rolandgeider

lib/widgets/add_exercise/preview_images.dart

@@ -2,8 +2,8 @@ import 'dart:io';
 
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
-
 import 'package:wger/providers/add_exercise.dart';
+
 import 'mixins/image_picker_mixin.dart';
 
 class PreviewExerciseImages extends StatelessWidget with ExerciseImagePickerMixin {
@@ -30,7 +30,7 @@ class PreviewExerciseImages extends StatelessWidget with ExerciseImagePickerMixi
                       padding: const EdgeInsets.all(3.0),
                       child: Container(
                         decoration: BoxDecoration(
-                          color: Colors.grey.withOpacity(0.5),
+                          color: Colors.grey.withValues(alpha: 0.5),
                           borderRadius: const BorderRadius.all(Radius.circular(20)),
                         ),
                         child: IconButton(