[release] Add release notes for 2.18.3
[release] Add release notes for 2.18.3

Change-Id: I291c49c56480786aa7ae4dff2b889e70157f62a0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/264420
Reviewed-by: Slava Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk