project.config: restrict submission to internal branches

Starting with Go 1.16, a new name for release branches will be used.
The same access controls that limit submission on release branches
to the release-managers group need to apply to the upcoming internal
branches too.

For golang.org/issue/36882.

Change-Id: Id6b34be93e05e15a409a83450a11829364cb6d33
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/283262
Reviewed-by: DO NOT USE <dmitshur@google.com>