build: upgrade Go to 1.26

[burmudar]

Updates the Go toolchain to 1.26

• Set the module minimum to Go 1.26.0.
• Use Go 1.26.7 in CI and the Docker builder.
• Update setup-go to v7.

Test plan

Ran against an isolated snapshot containing only this PR:

$ go test ./... -count=1
ok  github.com/sourcegraph/codenotify  0.936s

$ docker build --progress=plain -t codenotify:go-upgrade-check .
#15 naming to docker.io/library/codenotify:go-upgrade-check done
#15 DONE 0.6s

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff 8cb7277...72fe856.

Notify	File(s)
@nicksnyder	.github/workflows/build.yml Dockerfile go.mod