feat: update to sdk version 3 (#7)

BREAKING CHANGE: This removes Flutter 2.x
support. Users of the package need to update to
Flutter 3.x SDK versions to further use the package.

Author: mike-schmid

example/example.dart

@@ -1,7 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:generic_dropdown_widget/generic_dropdown_widget.dart';
 
-class Dropdown extends StatelessWidget {
+final class Dropdown extends StatelessWidget {
   const Dropdown({super.key});
 
   @override

lib/src/generic_dropdown.dart

@@ -78,7 +78,7 @@ enum DropdownDirection {
 /// content. The [contentBuilder] receives a "repaint" and a
 /// "close" callbacks, which can help updating the content
 /// or closing the content entirely.
-class GenericDropdown extends StatefulWidget {
+final class GenericDropdown extends StatefulWidget {
   /// The anchor of the dropdown. Defines
   /// on which point of the toggle the content
   /// will be anchored. Defaults to [DropdownAnchor.bottomLeft].
@@ -132,7 +132,7 @@ class GenericDropdown extends StatefulWidget {
   State<GenericDropdown> createState() => _GenericDropdownState();
 }
 
-class _GenericDropdownState extends State<GenericDropdown> {
+final class _GenericDropdownState extends State<GenericDropdown> {
   var _isOpen = false;
   OverlayEntry? _overlayEntry;
 

lib/src/generic_dropdown_config_provider.dart

@@ -12,7 +12,7 @@ import 'package:flutter/widgets.dart';
 /// at the wrong position. With this config, the dropdown can
 /// utilize the root storybook as root screen key to calculate
 /// the position of the content.
-class GenericDropdownConfigProvider extends InheritedWidget {
+final class GenericDropdownConfigProvider extends InheritedWidget {
   /// The key of the root screen that contains the dropdown.
   /// The screen is used to calculate the relative position
   /// of the content overlay to the toggle.

pubspec.yaml

@@ -7,7 +7,7 @@ issue_tracker: https://github.com/smartive/flutter-generic-dropdown-widget/issue
 repository: https://github.com/smartive/flutter-generic-dropdown-widget.git
 
 environment:
-  sdk: '>=2.19.3 <3.0.0'
+  sdk: '>=3.0.0 <4.0.0'
   flutter: '>=1.17.0'
 
 dependencies:

storybook/pubspec.yaml

@@ -4,7 +4,7 @@ publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 version: 1.0.0+1
 
 environment:
-  sdk: '>=2.19.3 <3.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 dependencies:
   flutter: