[infra] Bump version file on master to 2.3.0

This is a follow-up to:
https://dart-review.googlesource.com/c/sdk/+/99283

Change-Id: I67e613c879e8676f438b154dbcb2eac3d57a2a01
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/100741
Auto-Submit: Alexander Thomas <[email protected]>
Reviewed-by: Zach Anderson <[email protected]>
Commit-Queue: Alexander Thomas <[email protected]>