add a release note for cancellable web tasks

Author: Gingeh

release-content/migration-guides/web_tasks

@@ -0,0 +1,6 @@
+---
+title: Dropping Tasks in Web Builds
+pull_requests: [21795]
+---
+
+Dropping a `Task<T>` in a web build now cancels it, call `Task<T>::detach()` to keep the old behavior.

release-content/release-notes/web_tasks

@@ -0,0 +1,8 @@
+---
+title: Cancellable Web Tasks
+authors: ["@NthTensor","@Gingeh"]
+pull_requests: [21795]
+---
+
+Async Tasks can now be cancelled in web builds,
+this brings the web implementation's behavior into line with native builds.