project.config: grant submit access to the 1.26 release branch

For go.dev/issue/76474.

Change-Id: I16037f45ff6f2e7a5dc7aa8c6e740ecafb513b48
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/738540
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>