project.config: grant Forge Author Identity to approvers

This will allow approvers to create cherrypick CLs. See https://github.com/golang/go/issues/55841#issuecomment-1379347816.

Change-Id: I95dde41c842500ec7526ca2c1c78e16143ef70f1
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/461799
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>