revert go.mod — keep only Dockerfile bump

go 1.27 in go.mod triggers GOTOOLCHAIN download in CI runners (currently on 1.26.5).
Scope this PR to the Dockerfile rebuild only; go.mod update follows after
grpc v1.79.3/http2.TrailerPrefix compatibility is resolved.

go.mod

@@ -1,6 +1,6 @@
 module code.fritzlab.net/fritzlab/flock
 
-go 1.27
+go 1.26.1
 
 require (
 	github.com/containernetworking/cni v1.3.0