project.config: lock down release-branch.go1.23

This locks down the Go 1.23 release branch to behave like the other
release branches, as per the release process.

Change-Id: Ia02837cb6c4b29d1ba7d8a14b1f779367aa3f049
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/603937
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>