Remove deprecated core-js version from production builds

[zofskeez]

Several dependencies used by Vault UI were using a now deprecated version of core-js (2.6.11) which was causing vulnerability scans to fail. To resolve this, the following steps were taken to remove the deprecated version of core-js from production builds:

• updated ember-auto-import to 2.4.2 - apps are now responsible for requiring webpack
• removed ember-cli-element-closest-polyfill - Element.closest() is supported by all target browsers
• removed ember-maybe-import-regenerator - ES6 generators are supported by all target browsers
• removed @babel/polyfill use (now deprecated and uses deprecated core-js version) via ember-cli-babel config
• added eslint-plugin-compat for linting code compatibility against target browsers

Doing a search in the vendor file of a production ember build after the changes yields zero results for core-js and/or 2.6.11. Additionally, the build time dropped significantly from 6 minutes to just over 1 minute:

Before

After

It would be ideal to use @babel/plugin-transform-runtime with core-js 3 but I couldn't get it working with ember-cli-babel and the useBabelConfig option. Hopefully ember-cli-babel will be updated to remove the @babel/polyfill entirely and support the recommendation to use core-js directly.

Related ember-cli-babel issues:
emberjs/ember-cli-babel#443
emberjs/ember-cli-babel#444
emberjs/ember-cli-babel#417
emberjs/ember-cli-babel#446

[hashishaw]

Excellent description of changes and how you approached this. Thank you so much! 🚀