Move to Flutter 3.47, use the improvements that come with this (#64)

Reviewed-on: #64

lib/models/spec_versions_response.dart

@@ -1,24 +1,27 @@
 import "package:fast_immutable_collections/fast_immutable_collections.dart";
 import "package:freezed_annotation/freezed_annotation.dart";
+
 part "spec_versions_response.freezed.dart";
 part "spec_versions_response.g.dart";
 
-@freezed
-abstract class SpecVersionsResponse with _$SpecVersionsResponse {
-  const factory SpecVersionsResponse({
-    required IList<String> versions,
-    required UnstableFeatures unstableFeatures,
-  }) = _SpecVersionsResponse;
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const SpecVersionsResponse({
+  required final IList<String> versions,
+  required final UnstableFeatures unstableFeatures,
+}) with _$SpecVersionsResponse {
+  Map<String, Object?> toJson() => _$SpecVersionsResponseToJson(this);
 
   factory SpecVersionsResponse.fromJson(Map<String, Object?> json) =>
       _$SpecVersionsResponseFromJson(json);
 }
 
-@freezed
-abstract class UnstableFeatures with _$UnstableFeatures {
-  const factory UnstableFeatures({
-    @JsonKey(name: "uk.timedout.msc4494") @Default(false) bool msc4494,
-  }) = _UnstableFeatures;
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const UnstableFeatures({
+  @JsonKey(name: "uk.timedout.msc4494") final bool msc4494 = false,
+}) with _$UnstableFeatures {
+  Map<String, Object?> toJson() => _$UnstableFeaturesToJson(this);
 
   factory UnstableFeatures.fromJson(Map<String, Object?> json) =>
       _$UnstableFeaturesFromJson(json);