BuiltValue deprecation

BuiltValue should be deprecated, because Freezed does the same stuff and more, and much easier and more concise.
It was a good choice in 2018, but now it's undesirable for beginners to drag this outdated package into the modern projects.

BuiltValue has a strange ugly syntax with numerous getters and annotations:
```dart
abstract class Person implements Built {
static Serializer get serializer => _$personSerializer;

// Can never be null.
int get id;

@nullable
int get age;

@nullable
@BuiltValueField(wireName: 'first_name')
String get firstName;

@nullable
BuiltList get hobbies;

Person._();
factory Person([void Function(PersonBuilder) updates]) = _$Person;
}
```

While Freezed syntax is intuitive and much less verbose:
```dart
@freezed
class Person with _$Person {
const factory Person({
required String firstName,
required String lastName,
required int age,
}) = _Person;
}
```

Also it's great for BLoC states declaration:
```dart
@freezed
class Union with _$Union {
const factory Union.data(int value) = Data;
const factory Union.loading() = Loading;
const factory Union.error([String? message]) = Error;
}
```

BuiltValue also has a cumbersome copying through the Builder pattern:
```dart
var structuredData = new Account((b) => b
..user.update((b) => b
..name = 'John Smith')
..credentials.phone.update((b) => b
..country = Country.us
..number = '555 01234 567')
..node.left.left.left.account.update((b) => b
..user.name = 'John Smith II'
..user.nickname = 'Is lost in a tree')
..node.left.right.right.account.update((b) => b
..user.name = 'John Smith III'));
```

While Freezed supports a familiar copyWith method
```dart
newCompany = company.copyWith.director(name: 'John Doe');
```

You could continue supporting it, but I suggest you putting the statement "Prefer choosing Freezed" on a main page.