project.config: lock down release-branch.go1.26

Lock down the release branch to behave like the other release
branches, as per the release process. This effectively undoes
the short-term additional permission granted to all approvers
in CL 738540.

For go.dev/issue/76474.

Change-Id: I4e4eed53421dd9f0cb0fae5c7e3b26e7eb61ce64
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/740080
Reviewed-by: Carlos Amedee <carlos@golang.org>