Add CHANGELOG entry for type casting enum values by the subtype

Related to #35336.

The notable thing about #41516 is that unknown labels will no longer
match 0 on MySQL.

Matching unknown labels to 0 was not by design, but rather almost like a
bug, people should not rely on that behavior.

Closes #41595.

activerecord/CHANGELOG.md

@@ -1,3 +1,33 @@
+*   Type cast enum values by the original attribute type.
+
+    The notable thing about this change is that unknown labels will no longer match 0 on MySQL.
+
+    ```ruby
+    class Book < ActiveRecord::Base
+      enum :status, { proposed: 0, written: 1, published: 2 }
+    end
+    ```
+
+    Before:
+
+    ```ruby
+    # SELECT `books`.* FROM `books` WHERE `books`.`status` = 'prohibited' LIMIT 1
+    Book.find_by(status: :prohibited)
+    # => #<Book id: 1, status: "proposed", ...> (for mysql2 adapter)
+    # => ActiveRecord::StatementInvalid: PG::InvalidTextRepresentation: ERROR:  invalid input syntax for type integer: "prohibited" (for postgresql adapter)
+    # => nil (for sqlite3 adapter)
+    ```
+
+    After:
+
+    ```ruby
+    # SELECT `books`.* FROM `books` WHERE `books`.`status` IS NULL LIMIT 1
+    Book.find_by(status: :prohibited)
+    # => nil (for all adapters)
+    ```
+
+    *Ryuta Kamizono*
+
 *   Fixtures for `has_many :through` associations now load timestamps on join tables
 
     Given this fixture:

activerecord/test/cases/enum_test.rb

@@ -80,6 +80,7 @@ class EnumTest < ActiveRecord::TestCase
     assert_not_equal @book, Book.where.not(status: :published).first
     assert_equal @book, Book.where.not(status: :written).first
     assert_equal books(:ddd), Book.where(last_read: :forgotten).first
+    assert_nil Book.where(status: :prohibited).first
   end
 
   test "find via where with strings" do
@@ -90,6 +91,7 @@ class EnumTest < ActiveRecord::TestCase
     assert_not_equal @book, Book.where.not(status: "published").first
     assert_equal @book, Book.where.not(status: "written").first
     assert_equal books(:ddd), Book.where(last_read: "forgotten").first
+    assert_nil Book.where(status: "prohibited").first
   end
 
   test "find via where should be type casted" do