[release] Add release date for 2.16.0
[release] Add release date for 2.16.0

Fixes: https://github.com/dart-lang/sdk/issues/48328
Change-Id: Ic1c2a0e0fc2f256257aa6c0b84dc1d72e5347a44
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/231942
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