Update freezed to latest version

Every time I try to use the latest version of freezed I got this error:

```
Because spaces <0.5.0 depends on gap ^1.2.0 and no versions of spaces match >0.5.0, spaces <0.5.0-∞ or >0.5.0 requires gap ^1.2.0.
And because spaces 0.5.0 depends on freezed_annotation ^0.13.0-nullsafety.0 and my_flashcards depends on spaces any, either freezed_annotation ^0.13.0-nullsafety.0 or gap ^1.2.0.
So, because my_flashcards depends on both gap ^2.0.0 and freezed_annotation ^2.0.3, version solving failed.
pub get failed (1; So, because my_flashcards depends on both gap ^2.0.0 and freezed_annotation ^2.0.3, version solving failed.)
```