vendor: bump golang/mock to 1.6.0

Commit e3ed3ba7c9 bumps golang/mock to 1.6.0 in hack/tools.
It makes sense to keep its version in sync, so let's do the same
in the top level go.mod.

Generated by

	./hack/pin-dependency.sh github.com/golang/mock v1.6.0
	./hack/lint-dependencies.sh
	./hack/update-vendor.sh
	./hack/update-internal-modules.sh

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

vendor/github.com/golang/mock/gomock/controller.go

@@ -123,7 +123,7 @@ type Controller struct {
 // Controller.
 //
 // New in go1.14+, if you are passing a *testing.T into this function you no
-// longer need to call ctrl.Finish() in your test methods
+// longer need to call ctrl.Finish() in your test methods.
 func NewController(t TestReporter) *Controller {
 	h, ok := t.(TestHelper)
 	if !ok {
@@ -259,6 +259,9 @@ func (ctrl *Controller) Call(receiver interface{}, method string, args ...interf
 // Finish checks to see if all the methods that were expected to be called
 // were called. It should be invoked for each Controller. It is not idempotent
 // and therefore can only be invoked once.
+//
+// New in go1.14+, if you are passing a *testing.T into NewController function you no
+// longer need to call ctrl.Finish() in your test methods.
 func (ctrl *Controller) Finish() {
 	// If we're currently panicking, probably because this is a deferred call.
 	// This must be recovered in the deferred function.