Add CHANGELOG entry for 2.12.3

Specifies why a 2.12.3 patch is being released, but does not
contain a security advisory yet.

Change-Id: I07933586f49348dd16295c477932e4c7066b8414
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/195057
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Franklin Yow <frankyow@google.com>

CHANGELOG.md

@@ -144,6 +144,12 @@ Updated the Linter to `1.2.1`, which includes:
 
 [#44211]: https://github.com/dart-lang/sdk/issues/44211
 
+## 2.12.3 - 2021-04-12
+
+This is a patch release that fixes a vulnerability in `dart:html` related to
+DOM clobbering. Thanks again to **Vincenzo di Cicco** for finding and reporting
+this vulnerability.
+
 ## 2.12.2 - 2021-03-17
 
 This is a patch release that fixes crashes reported by Flutter 2 users (issue