Preparing pkgs:sse for 4.2.0 release (#2372)

4.2.0 is listed as a dep for DWDS https://github.com/dart-lang/webdev/blob/main/dwds/pubspec.yaml

Author: Markzipan

pkgs/sse/pubspec.yaml

@@ -1,5 +1,5 @@
 name: sse
-version: 4.2.0-wip
+version: 4.2.0
 description: >-
   Provides client and server functionality for setting up bi-directional
   communication through Server Sent Events (SSE) and corresponding POST