[release] Update CHANGELOG.md for 2.7.2
[release] Update CHANGELOG.md for 2.7.2

Change-Id: I1647e94a0371f1976a57b6db4906f499986a9b82
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139288
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
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