fix: bump go directive to 1.26.0

Aligns go.mod with the Dockerfile base image (golang:1.26.2).
Fixes setup-envtest@latest which now requires go >= 1.26.0.

Author: zdenko-kovac

go.mod

@@ -1,6 +1,6 @@
 module github.com/sap/cf-service-operator
 
-go 1.25.0
+go 1.26.0
 
 require (
 	github.com/cloudfoundry-community/go-cfclient/v3 v3.0.0-alpha.5