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

Reviewed-on: #64

lib/models/read_receipt.dart

@@ -1,17 +1,19 @@
 import "package:freezed_annotation/freezed_annotation.dart";
 import "package:nexus/models/epoch_date_time_converter.dart";
+
 part "read_receipt.freezed.dart";
 part "read_receipt.g.dart";
 
-@freezed
-abstract class ReadReceipt with _$ReadReceipt {
-  const factory ReadReceipt({
-    String? roomId,
-    required String userId,
-    String? threadId,
-    required String eventId,
-    @EpochDateTimeConverter() required DateTime timestamp,
-  }) = _ReadReceipt;
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const ReadReceipt({
+  final String? roomId,
+  required final String userId,
+  final String? threadId,
+  required final String eventId,
+  @EpochDateTimeConverter() required final DateTime timestamp,
+}) with _$ReadReceipt {
+  Map<String, Object?> toJson() => _$ReadReceiptToJson(this);
 
   factory ReadReceipt.fromJson(Map<String, Object?> json) =>
       _$ReadReceiptFromJson(json);