[release] Add 2.5.2 release notes missing on master
[release] Add 2.5.2 release notes missing on master

The 2.5.2 release notes were never copied from stable to master.

Change-Id: I270227bb1701fbbf1df2f22de6f72d022214175a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144561
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@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