[milestone] Add MILESTONE file This file is newly introduced into integration.git, so we should mirror it here. It will soon be required for proper generation of release version numbers. Bug: 70302 Change-Id: Ie594feeec9a5b7209feada286e76f76e54af081c Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/roller-tests/integration/+/486266 Fuchsia-Auto-Submit: Anthony Fandrianto <atyfto@google.com> Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com> Reviewed-by: Oliver Newman <olivernewman@google.com>