[release] Add release notes for 2.13.1
[release] Add release notes for 2.13.1

Change-Id: I896d729e1b6a1b38acc90235ed110ab5f906d6de
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200874
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