Deprecate whereNotNull from IterableNullableExtensions

[oprypin]

Dart SDK since 3.0 has an exact equivalent extension nonNulls in the core.

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• References Deprecate IterableExtensions that have exact equivalents in the SDK #330
• Closes rewrite whereNotNull without sync* #273
• Closes Provide faster implementation for whereNotNull #274

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[natebosch]

cc @bwilkerson - would it be feasible to add support in dart_fix.yaml to replace a method with no arguments with a field? I don't think we currently have a way to automate the fix we need:

import 'package:collection/collection.dart';

void main() {
  [].whereNotNull().join('');
}

Should become

void main() {
  [].nonNulls.join('');
}

The transform I think I want, but is currently an error:

  - title: 'Replace whereNotNull() with core nonNulls'
    date: 2024-01-25
    element:
      uris: ['collection.dart']
      method: 'whereNotNull'
      inExtension: 'IterableExtension'
    changes:
      - kind: 'replacedBy'
        newElement:
          uris: ['dart:core']
          field: 'nonNulls'
          inClass: 'NullableIterableExtensions'