Deprecate whereNotNull from IterableNullableExtensions

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

CHANGELOG.md

@@ -4,6 +4,9 @@
 - Shuffle `IterableExtension.sample` results.
 - Fix `mergeSort` when the runtime iterable generic is a subtype of the static
   generic.
+- Deprecate `whereNotNull()` from `IterableNullableExtension`. Use `nonNulls`
+  instead - this is an equivalent extension available in Dart core since
+  version 3.0.
 - Require Dart `^3.1.0`
 - Mark "mixin" classes as `mixin`.
 

lib/src/iterable_extensions.dart

@@ -611,6 +611,7 @@ extension IterableNullableExtension<T extends Object> on Iterable<T?> {
   /// of this iterable, in their original iteration order.
   ///
   /// For an `Iterable<X?>`, this method is equivalent to `.whereType<X>()`.
+  @Deprecated('Use .nonNulls instead.')
   Iterable<T> whereNotNull() sync* {
     for (var element in this) {
       if (element != null) yield element;