project.config: grant submit access to the 1.27 release branch

For go.dev/issue/79581

Change-Id: Id6ff9f3a7b78b0af5334a4aa22bb5ac7e3a71807
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/799482
Reviewed-by: Michael Pratt <mpratt@google.com>