docs: upgrade freezed class to match freezed version 3.* (#360)

Author: Xsantosh

bricks/stac_parser/__brick__/{{#snakecase}}stac_{{name}}{{/snakecase}}/{{#snakecase}}stac_{{name}}{{/snakecase}}.dart

@@ -4,7 +4,7 @@ part '{{#snakeCase}}stac_{{name}}{{/snakeCase}}.freezed.dart';
 part '{{#snakeCase}}stac_{{name}}{{/snakeCase}}.g.dart';
 
 @freezed
-class Stac{{#pascalCase}}{{name}}{{/pascalCase}} with _$Stac{{#pascalCase}}{{name}}{{/pascalCase}} {
+abstract class Stac{{#pascalCase}}{{name}}{{/pascalCase}} with _$Stac{{#pascalCase}}{{name}}{{/pascalCase}} {
   const factory Stac{{#pascalCase}}{{name}}{{/pascalCase}}() = _Stac{{#pascalCase}}{{name}}{{/pascalCase}};
 
   factory Stac{{#pascalCase}}{{name}}{{/pascalCase}}.fromJson(Map<String, dynamic> json) => _$Stac{{#pascalCase}}{{name}}{{/pascalCase}}FromJson(json);

website/docs/concepts/action_parsers.md

@@ -36,7 +36,7 @@ For this JSON Structure, we can create a data class to represent the custom acti
 
 ```dart
 @freezed
-class PrintAction with _$PrintAction {
+abstract class PrintAction with _$PrintAction {
   const factory PrintAction({
     required String message,
   }) = _PrintAction;

website/docs/concepts/parsers.md

@@ -45,7 +45,7 @@ Here we are using the freezed package to create the data class. But you can use
 
 ```dart
 @freezed
-class CustomButton with _$CustomButton {
+abstract class CustomButton with _$CustomButton {
   const factory CustomButton({
     required String text,
     required String color,