Update to Istio 1.4 and retire 1.2

[nak3]

In what area(s)?

/area networking
/area test-and-release

Describe the feature

• As per title, this is a tracking ticket to update to Istio 1.4 and retire 1.2.
• Istio 1.4 was released and 1.2 ends on December 13th/2019

[nak3]

Istio 1.4 introduced Regex EngineChanges so most of E2E tests fail due to exceed the size of regex bytes in VirtualService.

e.g:

    match:
    - authority:
        regex: ^service-to-service-call-via-activator-both-disabled-rlzfmjaj\.serving-tests\.example\.com(?::\d{1,5})?$

Then, we will get an error.
{"level":"error","ts":"2019-11-16T08:13:54.839Z","logger":"istiocontroller.ingress-controller","caller":"controller/controller.go:368","msg":"Reconcile error","commit":"f961c64","knative.dev/controller":"ingress-controller","error":"failed to create VirtualService: admission webhook \"pilot.validation.istio.io\" denied the request: configuration is invalid: regex match '^service-to-service-call-via-activator-both-disabled-rlzfmjaj\\.serving-tests(\\.svc(\\.cluster\\.local)?)?(?::\\d{1,5})?$' cannot be greater than 100 bytes","stacktrace":"knative.dev/serving/vendor/knative.dev/pkg/controller.(*Impl).handleErr\n\t/home/knakayam/.go/src/knative.dev/serving/vendor/knative.dev/pkg/controller/controller.go:368\nknative.dev/serving/vendor/knative.dev/pkg/controller.(*Impl).processNextWorkItem\n\t/home/knakayam/.go/src/knative.dev/serving/vendor/knative.dev/pkg/controller/controller.go:354\nknative.dev/serving/vendor/knative.dev/pkg/controller.(*Impl).Run.func2\n\t/home/knakayam/.go/src/knative.dev/serving/vendor/knative.dev/pkg/controller/controller.go:302"}

We need to address this.

[nak3]

I've opened #6058 for regex size issue.