Bumped webhook API defaults to v4

refs https://github.com/TryGhost/Ghost/issues/12716

- The default API in Ghost v4 will be `'v4'`. When a new webhook is created or any data passes through serialization it should assume `v4` as a fallback if not specified.

core/server/models/webhook.js

@@ -8,7 +8,7 @@ Webhook = ghostBookshelf.Model.extend({
 
     defaults() {
         return {
-            api_version: 'v3',
+            api_version: 'v4',
             status: 'available'
         };
     },

core/server/services/webhooks/serialize.js

@@ -3,7 +3,7 @@ module.exports = (event, model) => {
     const {sequence} = require('@tryghost/promise');
     const api = require('../../api');
 
-    const apiVersion = model.get('api_version') || 'v3';
+    const apiVersion = model.get('api_version') || 'v4';
 
     const resourceName = event.match(/(\w+)\./)[1];
     const docName = `${resourceName}s`;