update breaking change policy
update breaking change policy

Closes https://github.com/dart-lang/sdk/pull/48463

GitOrigin-RevId: c711f1456983b39f2b76ab1724e7373da45490ed
Change-Id: I1208cb437cd41939ab4d3c2842bb681cb1be3f33
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/234104
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Commit-Queue: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@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