Bump Go to 1.26.3 in go.mod

`setup-envtest@latest` (resolved to v0.24.1) requires Go 1.26 and the CI
pipeline installs the Go version requested in `go.mod`.

Author: simu

go.mod

@@ -1,6 +1,6 @@
 module github.com/appuio/scheduler-canary-controller
 
-go 1.25.6
+go 1.26.3
 
 require (
 	github.com/go-logr/logr v1.4.3