Updated eslint ecma version to 2020

refs TryGhost/Ghost@bb1a570

- Main Ghost repository is already using this syntax. Needed the nulllish syntax in one of the function refactors really badly - otherwise would create lots of unnecessary code, so decided to enable it now

Author: naz

packages/admin-api-schema/.eslintrc.js

@@ -1,4 +1,7 @@
 module.exports = {
+    parserOptions: {
+        ecmaVersion: 2020
+    },
     plugins: ['ghost'],
     extends: [
         'plugin:ghost/node'