[dart:html] Add changelog note for EventStreamSubscription.cancel
[dart:html] Add changelog note for EventStreamSubscription.cancel

Closes https://github.com/dart-lang/sdk/issues/41653

Change-Id: I0e5cac66a3a80c3c753d740bf8bf90f68addaf6d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/146880
Commit-Queue: Srujan Gaddam <4175f2f2dd3cde6c8ffb10fc9674dc09abf813fe@google.com>
Reviewed-by: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@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