Update the template for generated VCL to use version 4.1.

And update the "golden" files for testing generated templates.

pkg/varnish/vcl/testdata/backendcfg.golden

-vcl 4.0;
+vcl 4.1;
 
 import std;
 import directors;

pkg/varnish/vcl/testdata/custom_vcl.golden

-vcl 4.0;
+vcl 4.1;
 
 import std;
 import directors;

pkg/varnish/vcl/testdata/ingress_shard.golden

-vcl 4.0;
+vcl 4.1;
 
 import std;
 import directors;

pkg/varnish/vcl/testdata/ingressrule.golden

-vcl 4.0;
+vcl 4.1;
 
 import std;
 import directors;

pkg/varnish/vcl/testdata/rewrite_auth_delete.golden

-vcl 4.0;
+vcl 4.1;
 
 import std;
 import directors;

pkg/varnish/vcl/vcl.tmpl

-vcl 4.0;
+vcl 4.1;
 
 import std;
 import directors;