Add CHANGELOG entry for 2.12.3
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 <4175f2f2dd3cde6c8ffb10fc9674dc09abf813fe@google.com>
Reviewed-by: Franklin Yow <4b0949275c52382c90c87677b9ea8aadf3be0e3f@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