Add older security advisories to changelog
Add older security advisories to changelog

Change-Id: I79a6d2065b7b2043401ee648a7238b8563729aff
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/232084
Reviewed-by: 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