breaking change to pubsub client

[Jonathan Amsterdam]

One change to the cloud.google.com/go/pubsub package:

Subscription.ModifyPushConfig is replaced with Subscription.Update.

Replace

    sub.ModifyPushConfig(ctx, pubsub.PushConfig{Endpoint: "https://example.com/push"})

with

    sub.Update(ctx, pubsub.SubscriptionConfigToUpdate{

PushConfig: &pubsub.PushConfig{Endpoint: "https://example.com/push"},

    })