project.config: grant submit access to the 1.25 release branch

For go.dev/issue/73829.

Change-Id: If273b35308fac574806bbfd52df7a1dbfa375f93
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/689995
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>