Merge branch '33315-es6-findindex-support' into 'master'

Add Array `findIndex` support

Closes #33315

See merge request !11961

app/assets/javascripts/commons/polyfills.js

 // ECMAScript polyfills
 import 'core-js/fn/array/find';
+import 'core-js/fn/array/find-index';
 import 'core-js/fn/array/from';
 import 'core-js/fn/array/includes';
 import 'core-js/fn/object/assign';