style: fix lints

Author: deandreamatias

lib/src/form_builder_image_picker.dart

@@ -304,7 +304,7 @@ class FormBuilderImagePicker extends FormBuilderFieldDecoration<List<dynamic>> {
                         child: Container(
                           margin: const EdgeInsets.all(3),
                           decoration: BoxDecoration(
-                            color: Colors.grey.withOpacity(.7),
+                            color: Colors.grey.withValues(alpha: .7),
                             shape: BoxShape.circle,
                           ),
                           alignment: Alignment.center,