[release] Add 2.15.1 to the changelog
[release] Add 2.15.1 to the changelog

Change-Id: I16329c5526533bde598c0557bf3d538b42e19046
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/223802
Reviewed-by: Michael Thomsen <5977a55c75bb4612004451a56b0b4c22d9d3311d@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