Add 2.15 release date
Add 2.15 release date

Fixes https://github.com/dart-lang/sdk/issues/47909

Bug: https://github.com/dart-lang/sdk/issues/47909
Change-Id: I7a2c81ba3888b52db2f69df36e17c58de7522179
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/223424
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